I created a table x with PK x_key_id as NUMBER. I also created a sequence to be able to increment this key.

Now, we have upgraded to 12C and I want to take advantage of oracle's embedded "sequence"

Creating a new table y with the surrogate key as:
x_key_id NUMBER GENERATED AS IDENTITY (START WITH 1 INCREMENT BY 1 NOCACHE) is no problem.

How do I alter the older tables to add the generated as identity on the PK? The only way I can think of is to rename the table, create a new table, move the data into new table and drop the old renamed table. I sure hope there is an easier way to do it. Thanks for any help.

create table t (
  x int
);

create sequence s;
alter table t modify x default s.nextval;

insert into t values (default);
select * from t;

X  
1 

alter table t add ( y int generated as identity );

insert into t values (default, default);
select * from t;

X  Y  
1  1  
2  2