I don't know of a command to reset these to the defaults.

Note that the default initial for partitions is 8Mb, regardless of the tablespace settings. You can override this for new partitions by changing the table's default attributes.

I'm also struggling to understand what the issue is here. It's easy enough to write a script to move all tables/partitions, setting the initial value as needed. I don't see how re-creating these is better as there's the risk of losing indexes, triggers, grants, etc. in the process.

That said, as you're on 19c there is a workaround of sorts. You can change the partitioning scheme for a table. So you could alter the table to use a different partition method, then back to the one you want. This will pick up the default (8Mb unless you've overridden it) in the process.

For example:

create table t (
  c1 int
) partition by range ( c1 ) (
    partition p1 values less than ( 10 ) storage ( initial 1k ),
    partition p2 values less than ( 20 ) storage ( initial 10k ),
    partition p3 values less than ( 30 ) storage ( initial 100k ),
    partition p4 values less than ( 40 ) storage ( initial 1000k )
  );
  
select partition_name, initial_extent
from   user_tab_partitions
where  table_name = 'T';

PARTITION_NAME       INITIAL_EXTENT    
P1                            16384 
P2                            16384 
P3                           106496 
P4                          1024000 
  
alter table t 
  modify partition by hash ( c1 )
  partitions 1;

select partition_name, initial_extent
from   user_tab_partitions
where  table_name = 'T';

PARTITION_NAME       INITIAL_EXTENT    
SYS_P45145                  8388608 

alter table t 
  modify default attributes
  storage ( initial 10m );

alter table t 
  modify partition by range ( c1 ) (
    partition p1 values less than ( 10 ),
    partition p2 values less than ( 20 ),
    partition p3 values less than ( 30 ),
    partition p4 values less than ( 40 )
  );
  
select partition_name, initial_extent
from   user_tab_partitions
where  table_name = 'T';

PARTITION_NAME       INITIAL_EXTENT    
P1                         10485760 
P2                         10485760 
P3                         10485760 
P4                         10485760 

Again, I don't see how this is any better/easier than moving the partitions as needed.