When a table is composite partitioned (ie, is partitioned and each partition has subpartitions), the partitions are "logical". They dont have any rows so to speak - they simply are a logical grouping of the subpartitions which *do* have rows (ie, are physical segments on the database).
So converting a partitioned table to a subpartitioned table will mean physically moving the rows.
The easiest way to do this would be with DBMS_REDEFINITION. An example is below, but refer to the documentation for complete usage
SQL> create table t ( x date , y int, z int )
2 partition by range ( x )
3 interval ( numtodsinterval(1,'DAY'))
4 (
5 partition p1 values less than ( date '2016-01-01' )
6 );
Table created.
SQL>
SQL> insert into t
2 select date '2016-01-01' + trunc(rownum/50),
3 rownum,
4 rownum
5 from dual
6 connect by level <= 4000;
4000 rows created.
SQL>
SQL> alter table T add primary key ( y ) ;
Table altered.
SQL>
SQL> exec dbms_stats.gather_table_Stats('','t')
PL/SQL procedure successfully completed.
SQL>
SQL> select partition_name, num_rows
2 from user_tab_partitions
3 where table_name = 'T'
4 order by partition_position;
PARTITION_NAME NUM_ROWS
------------------------------ ----------
P1 0
SYS_P15705 49
SYS_P15706 50
SYS_P15707 50
SYS_P15708 50
SYS_P15709 50
SYS_P15710 50
SYS_P15711 50
SYS_P15712 50
SYS_P15713 50
SYS_P15714 50
SYS_P15715 50
SYS_P15716 50
SYS_P15717 50
SYS_P15718 50
SYS_P15719 50
SYS_P15720 50
SYS_P15721 50
SYS_P15722 50
SYS_P15723 50
SYS_P15724 50
SYS_P15725 50
SYS_P15726 50
SYS_P15727 50
SYS_P15728 50
SYS_P15729 50
SYS_P15730 50
SYS_P15731 50
SYS_P15732 50
SYS_P15733 50
SYS_P15734 50
SYS_P15735 50
SYS_P15736 50
SYS_P15737 50
SYS_P15738 50
SYS_P15739 50
SYS_P15740 50
SYS_P15741 50
SYS_P15742 50
SYS_P15743 50
SYS_P15744 50
SYS_P15745 50
SYS_P15746 50
SYS_P15747 50
SYS_P15748 50
SYS_P15749 50
SYS_P15750 50
SYS_P15751 50
SYS_P15752 50
SYS_P15753 50
SYS_P15754 50
SYS_P15755 50
SYS_P15756 50
SYS_P15757 50
SYS_P15758 50
SYS_P15759 50
SYS_P15760 50
SYS_P15761 50
SYS_P15762 50
SYS_P15763 50
SYS_P15764 50
SYS_P15765 50
SYS_P15766 50
SYS_P15767 50
SYS_P15768 50
SYS_P15769 50
SYS_P15770 50
SYS_P15771 50
SYS_P15772 50
SYS_P15773 50
SYS_P15774 50
SYS_P15775 50
SYS_P15776 50
SYS_P15777 50
SYS_P15778 50
SYS_P15779 50
SYS_P15780 50
SYS_P15781 50
SYS_P15782 50
SYS_P15783 50
SYS_P15784 50
SYS_P15785 1
82 rows selected.
SQL>
SQL> create table new_t ( x date , y int, z int )
2 partition by range ( x ) interval ( numtodsinterval(1,'DAY'))
3 subpartition by hash (y)
4 subpartition template
5 ( subpartition a, subpartition b, subpartition c, subpartition d )
6 (
7 partition p1 values less than ( date '2016-01-01' )
8 );
Table created.
SQL>
SQL> alter table new_T add primary key ( y ) ;
Table altered.
SQL>
SQL> EXEC DBMS_REDEFINITION.start_redef_table(user, 'T', 'NEW_T');
PL/SQL procedure successfully completed.
SQL>
SQL> EXEC DBMS_REDEFINITION.finish_redef_table(user, 'T', 'NEW_T');
PL/SQL procedure successfully completed.
SQL> exec dbms_stats.gather_table_Stats('','t',granularity=>'ALL')
PL/SQL procedure successfully completed.
SQL> select partition_name, subpartition_name, num_rows
2 from user_tab_subpartitions
3 where table_name = 'T';
PARTITION_NAME SUBPARTITION_NAME NUM_ROWS
------------------------------ ------------------------------ ----------
P1 P1_A 0
P1 P1_B 0
P1 P1_C 0
P1 P1_D 0
SYS_P15790 SYS_SUBP15786 8
SYS_P15790 SYS_SUBP15787 13
SYS_P15790 SYS_SUBP15788 13
SYS_P15790 SYS_SUBP15789 15
SYS_P15795 SYS_SUBP15791 12
SYS_P15795 SYS_SUBP15792 15
SYS_P15795 SYS_SUBP15793 11
SYS_P15795 SYS_SUBP15794 12
SYS_P15800 SYS_SUBP15796 9
SYS_P15800 SYS_SUBP15797 10
SYS_P15800 SYS_SUBP15798 21
SYS_P15800 SYS_SUBP15799 10
SYS_P15805 SYS_SUBP15801 14
SYS_P15805 SYS_SUBP15802 20
SYS_P15805 SYS_SUBP15803 8
SYS_P15805 SYS_SUBP15804 8
SYS_P15810 SYS_SUBP15806 10
SYS_P15810 SYS_SUBP15807 15
SYS_P15810 SYS_SUBP15808 15
SYS_P15810 SYS_SUBP15809 10
SYS_P15815 SYS_SUBP15811 13
SYS_P15815 SYS_SUBP15812 12
SYS_P15815 SYS_SUBP15813 10
SYS_P15815 SYS_SUBP15814 15
SYS_P15820 SYS_SUBP15816 12
SYS_P15820 SYS_SUBP15817 7
SYS_P15820 SYS_SUBP15818 17
SYS_P15820 SYS_SUBP15819 14
SYS_P15825 SYS_SUBP15821 11
SYS_P15825 SYS_SUBP15822 16
SYS_P15825 SYS_SUBP15823 13
SYS_P15825 SYS_SUBP15824 10
SYS_P15830 SYS_SUBP15826 14
SYS_P15830 SYS_SUBP15827 14
SYS_P15830 SYS_SUBP15828 11
SYS_P15830 SYS_SUBP15829 11
SYS_P15835 SYS_SUBP15831 11
SYS_P15835 SYS_SUBP15832 12
SYS_P15835 SYS_SUBP15833 13
SYS_P15835 SYS_SUBP15834 14
SYS_P15840 SYS_SUBP15836 13
SYS_P15840 SYS_SUBP15837 9
SYS_P15840 SYS_SUBP15838 16
SYS_P15840 SYS_SUBP15839 12
SYS_P15845 SYS_SUBP15841 16
SYS_P15845 SYS_SUBP15842 7
SYS_P15845 SYS_SUBP15843 13
SYS_P15845 SYS_SUBP15844 14
SYS_P15850 SYS_SUBP15846 10
SYS_P15850 SYS_SUBP15847 13
SYS_P15850 SYS_SUBP15848 11
SYS_P15850 SYS_SUBP15849 16
SYS_P15855 SYS_SUBP15851 6
SYS_P15855 SYS_SUBP15852 15
SYS_P15855 SYS_SUBP15853 13
SYS_P15855 SYS_SUBP15854 16
SYS_P15860 SYS_SUBP15856 18
SYS_P15860 SYS_SUBP15857 8
SYS_P15860 SYS_SUBP15858 12
SYS_P15860 SYS_SUBP15859 12
SYS_P15865 SYS_SUBP15861 7
SYS_P15865 SYS_SUBP15862 12
SYS_P15865 SYS_SUBP15863 13
SYS_P15865 SYS_SUBP15864 18
SYS_P15870 SYS_SUBP15866 14
SYS_P15870 SYS_SUBP15867 10
SYS_P15870 SYS_SUBP15868 16
SYS_P15870 SYS_SUBP15869 10
SYS_P15875 SYS_SUBP15871 15
SYS_P15875 SYS_SUBP15872 11
SYS_P15875 SYS_SUBP15873 11
SYS_P15875 SYS_SUBP15874 13
SYS_P15880 SYS_SUBP15876 7
SYS_P15880 SYS_SUBP15877 14
SYS_P15880 SYS_SUBP15878 11
SYS_P15880 SYS_SUBP15879 18
SYS_P15885 SYS_SUBP15881 13
SYS_P15885 SYS_SUBP15882 11
SYS_P15885 SYS_SUBP15883 13
SYS_P15885 SYS_SUBP15884 13
SYS_P15890 SYS_SUBP15886 12
SYS_P15890 SYS_SUBP15887 11
SYS_P15890 SYS_SUBP15888 13
SYS_P15890 SYS_SUBP15889 14
SYS_P15895 SYS_SUBP15891 9
SYS_P15895 SYS_SUBP15892 12
SYS_P15895 SYS_SUBP15893 16
SYS_P15895 SYS_SUBP15894 13
SYS_P15900 SYS_SUBP15896 18
SYS_P15900 SYS_SUBP15897 8
SYS_P15900 SYS_SUBP15898 9
SYS_P15900 SYS_SUBP15899 15
PARTITION_NAME SUBPARTITION_NAME NUM_ROWS
------------------------------ ------------------------------ ----------
SYS_P15905 SYS_SUBP15901 18
SYS_P15905 SYS_SUBP15902 11
SYS_P15905 SYS_SUBP15903 10
SYS_P15905 SYS_SUBP15904 11
SYS_P15910 SYS_SUBP15906 11
SYS_P15910 SYS_SUBP15907 14
SYS_P15910 SYS_SUBP15908 14
SYS_P15910 SYS_SUBP15909 11
SYS_P15915 SYS_SUBP15911 9
SYS_P15915 SYS_SUBP15912 11
SYS_P15915 SYS_SUBP15913 16
SYS_P15915 SYS_SUBP15914 14
SYS_P15920 SYS_SUBP15916 15
SYS_P15920 SYS_SUBP15917 12
SYS_P15920 SYS_SUBP15918 9
SYS_P15920 SYS_SUBP15919 14
SYS_P15925 SYS_SUBP15921 18
SYS_P15925 SYS_SUBP15922 8
SYS_P15925 SYS_SUBP15923 11
SYS_P15925 SYS_SUBP15924 13
SYS_P15930 SYS_SUBP15926 12
SYS_P15930 SYS_SUBP15927 16
SYS_P15930 SYS_SUBP15928 9
SYS_P15930 SYS_SUBP15929 13
SYS_P15935 SYS_SUBP15931 11
SYS_P15935 SYS_SUBP15932 17
SYS_P15935 SYS_SUBP15933 14
SYS_P15935 SYS_SUBP15934 8
SYS_P15940 SYS_SUBP15936 18
SYS_P15940 SYS_SUBP15937 12
SYS_P15940 SYS_SUBP15938 8
SYS_P15940 SYS_SUBP15939 12
SYS_P15945 SYS_SUBP15941 10
SYS_P15945 SYS_SUBP15942 16
SYS_P15945 SYS_SUBP15943 16
SYS_P15945 SYS_SUBP15944 8
SYS_P15950 SYS_SUBP15946 16
SYS_P15950 SYS_SUBP15947 12
SYS_P15950 SYS_SUBP15948 12
SYS_P15950 SYS_SUBP15949 10
SYS_P15955 SYS_SUBP15951 9
SYS_P15955 SYS_SUBP15952 18
SYS_P15955 SYS_SUBP15953 14
SYS_P15955 SYS_SUBP15954 9
SYS_P15960 SYS_SUBP15956 11
SYS_P15960 SYS_SUBP15957 18
SYS_P15960 SYS_SUBP15958 12
SYS_P15960 SYS_SUBP15959 9
SYS_P15965 SYS_SUBP15961 11
SYS_P15965 SYS_SUBP15962 11
SYS_P15965 SYS_SUBP15963 14
SYS_P15965 SYS_SUBP15964 14
SYS_P15970 SYS_SUBP15966 11
SYS_P15970 SYS_SUBP15967 16
SYS_P15970 SYS_SUBP15968 15
SYS_P15970 SYS_SUBP15969 8
SYS_P15975 SYS_SUBP15971 17
SYS_P15975 SYS_SUBP15972 12
SYS_P15975 SYS_SUBP15973 15
SYS_P15975 SYS_SUBP15974 6
SYS_P15980 SYS_SUBP15976 11
SYS_P15980 SYS_SUBP15977 14
SYS_P15980 SYS_SUBP15978 13
SYS_P15980 SYS_SUBP15979 12
SYS_P15985 SYS_SUBP15981 11
SYS_P15985 SYS_SUBP15982 11
SYS_P15985 SYS_SUBP15983 11
SYS_P15985 SYS_SUBP15984 17
SYS_P15990 SYS_SUBP15986 9
SYS_P15990 SYS_SUBP15987 10
SYS_P15990 SYS_SUBP15988 20
SYS_P15990 SYS_SUBP15989 11
SYS_P15995 SYS_SUBP15991 6
SYS_P15995 SYS_SUBP15992 13
SYS_P15995 SYS_SUBP15993 13
SYS_P15995 SYS_SUBP15994 18
SYS_P16000 SYS_SUBP15996 11
SYS_P16000 SYS_SUBP15997 19
SYS_P16000 SYS_SUBP15998 10
SYS_P16000 SYS_SUBP15999 10
SYS_P16005 SYS_SUBP16001 15
SYS_P16005 SYS_SUBP16002 13
SYS_P16005 SYS_SUBP16003 10
SYS_P16005 SYS_SUBP16004 12
SYS_P16010 SYS_SUBP16006 10
SYS_P16010 SYS_SUBP16007 11
SYS_P16010 SYS_SUBP16008 14
SYS_P16010 SYS_SUBP16009 15
SYS_P16015 SYS_SUBP16011 11
SYS_P16015 SYS_SUBP16012 13
SYS_P16015 SYS_SUBP16013 13
SYS_P16015 SYS_SUBP16014 13
SYS_P16020 SYS_SUBP16016 21
SYS_P16020 SYS_SUBP16017 5
SYS_P16020 SYS_SUBP16018 14
SYS_P16020 SYS_SUBP16019 10
PARTITION_NAME SUBPARTITION_NAME NUM_ROWS
------------------------------ ------------------------------ ----------
SYS_P16025 SYS_SUBP16021 11
SYS_P16025 SYS_SUBP16022 12
SYS_P16025 SYS_SUBP16023 10
SYS_P16025 SYS_SUBP16024 17
SYS_P16030 SYS_SUBP16026 15
SYS_P16030 SYS_SUBP16027 12
SYS_P16030 SYS_SUBP16028 9
SYS_P16030 SYS_SUBP16029 14
SYS_P16035 SYS_SUBP16031 13
SYS_P16035 SYS_SUBP16032 8
SYS_P16035 SYS_SUBP16033 11
SYS_P16035 SYS_SUBP16034 18
SYS_P16040 SYS_SUBP16036 12
SYS_P16040 SYS_SUBP16037 11
SYS_P16040 SYS_SUBP16038 11
SYS_P16040 SYS_SUBP16039 16
SYS_P16045 SYS_SUBP16041 11
SYS_P16045 SYS_SUBP16042 11
SYS_P16045 SYS_SUBP16043 17
SYS_P16045 SYS_SUBP16044 11
SYS_P16050 SYS_SUBP16046 10
SYS_P16050 SYS_SUBP16047 9
SYS_P16050 SYS_SUBP16048 17
SYS_P16050 SYS_SUBP16049 14
SYS_P16055 SYS_SUBP16051 15
SYS_P16055 SYS_SUBP16052 8
SYS_P16055 SYS_SUBP16053 19
SYS_P16055 SYS_SUBP16054 8
SYS_P16060 SYS_SUBP16056 20
SYS_P16060 SYS_SUBP16057 11
SYS_P16060 SYS_SUBP16058 10
SYS_P16060 SYS_SUBP16059 9
SYS_P16065 SYS_SUBP16061 10
SYS_P16065 SYS_SUBP16062 14
SYS_P16065 SYS_SUBP16063 14
SYS_P16065 SYS_SUBP16064 12
SYS_P16070 SYS_SUBP16066 12
SYS_P16070 SYS_SUBP16067 12
SYS_P16070 SYS_SUBP16068 14
SYS_P16070 SYS_SUBP16069 12
SYS_P16075 SYS_SUBP16071 11
SYS_P16075 SYS_SUBP16072 16
SYS_P16075 SYS_SUBP16073 14
SYS_P16075 SYS_SUBP16074 9
SYS_P16080 SYS_SUBP16076 12
SYS_P16080 SYS_SUBP16077 15
SYS_P16080 SYS_SUBP16078 10
SYS_P16080 SYS_SUBP16079 13
SYS_P16085 SYS_SUBP16081 10
SYS_P16085 SYS_SUBP16082 13
SYS_P16085 SYS_SUBP16083 15
SYS_P16085 SYS_SUBP16084 12
SYS_P16090 SYS_SUBP16086 21
SYS_P16090 SYS_SUBP16087 9
SYS_P16090 SYS_SUBP16088 8
SYS_P16090 SYS_SUBP16089 12
SYS_P16095 SYS_SUBP16091 11
SYS_P16095 SYS_SUBP16092 16
SYS_P16095 SYS_SUBP16093 12
SYS_P16095 SYS_SUBP16094 11
SYS_P16100 SYS_SUBP16096 15
SYS_P16100 SYS_SUBP16097 8
SYS_P16100 SYS_SUBP16098 15
SYS_P16100 SYS_SUBP16099 12
SYS_P16105 SYS_SUBP16101 13
SYS_P16105 SYS_SUBP16102 12
SYS_P16105 SYS_SUBP16103 11
SYS_P16105 SYS_SUBP16104 14
SYS_P16110 SYS_SUBP16106 15
SYS_P16110 SYS_SUBP16107 13
SYS_P16110 SYS_SUBP16108 10
SYS_P16110 SYS_SUBP16109 12
SYS_P16115 SYS_SUBP16111 14
SYS_P16115 SYS_SUBP16112 11
SYS_P16115 SYS_SUBP16113 15
SYS_P16115 SYS_SUBP16114 10
SYS_P16120 SYS_SUBP16116 15
SYS_P16120 SYS_SUBP16117 12
SYS_P16120 SYS_SUBP16118 14
SYS_P16120 SYS_SUBP16119 9
SYS_P16125 SYS_SUBP16121 11
SYS_P16125 SYS_SUBP16122 11
SYS_P16125 SYS_SUBP16123 15
SYS_P16125 SYS_SUBP16124 13
SYS_P16130 SYS_SUBP16126 12
SYS_P16130 SYS_SUBP16127 18
SYS_P16130 SYS_SUBP16128 10
SYS_P16130 SYS_SUBP16129 10
SYS_P16135 SYS_SUBP16131 14
SYS_P16135 SYS_SUBP16132 16
SYS_P16135 SYS_SUBP16133 8
SYS_P16135 SYS_SUBP16134 12
SYS_P16140 SYS_SUBP16136 11
SYS_P16140 SYS_SUBP16137 13
SYS_P16140 SYS_SUBP16138 13
SYS_P16140 SYS_SUBP16139 13
PARTITION_NAME SUBPARTITION_NAME NUM_ROWS
------------------------------ ------------------------------ ----------
SYS_P16145 SYS_SUBP16141 11
SYS_P16145 SYS_SUBP16142 7
SYS_P16145 SYS_SUBP16143 13
SYS_P16145 SYS_SUBP16144 19
SYS_P16150 SYS_SUBP16146 11
SYS_P16150 SYS_SUBP16147 11
SYS_P16150 SYS_SUBP16148 12
SYS_P16150 SYS_SUBP16149 16
SYS_P16155 SYS_SUBP16151 14
SYS_P16155 SYS_SUBP16152 14
SYS_P16155 SYS_SUBP16153 13
SYS_P16155 SYS_SUBP16154 9
SYS_P16160 SYS_SUBP16156 12
SYS_P16160 SYS_SUBP16157 10
SYS_P16160 SYS_SUBP16158 15
SYS_P16160 SYS_SUBP16159 13
SYS_P16165 SYS_SUBP16161 9
SYS_P16165 SYS_SUBP16162 16
SYS_P16165 SYS_SUBP16163 13
SYS_P16165 SYS_SUBP16164 12
SYS_P16170 SYS_SUBP16166 15
SYS_P16170 SYS_SUBP16167 13
SYS_P16170 SYS_SUBP16168 10
SYS_P16170 SYS_SUBP16169 12
SYS_P16175 SYS_SUBP16171 14
SYS_P16175 SYS_SUBP16172 13
SYS_P16175 SYS_SUBP16173 10
SYS_P16175 SYS_SUBP16174 13
SYS_P16180 SYS_SUBP16176 9
SYS_P16180 SYS_SUBP16177 15
SYS_P16180 SYS_SUBP16178 13
SYS_P16180 SYS_SUBP16179 13
SYS_P16185 SYS_SUBP16181 9
SYS_P16185 SYS_SUBP16182 16
SYS_P16185 SYS_SUBP16183 17
SYS_P16185 SYS_SUBP16184 8
SYS_P16190 SYS_SUBP16186 0
SYS_P16190 SYS_SUBP16187 1
SYS_P16190 SYS_SUBP16188 0
SYS_P16190 SYS_SUBP16189 0
328 rows selected.
SQL>