SQL> insert into t1 (id, val) select null, round(dbms_random.value*1000, 0) from dual connect by level < 1001;

1000 rows inserted


SQL> 
SQL> merge into t1 using (select rownum-1 id from dual connect by level< 1002) new_id on (t1.val = new_id.id)
  2   when matched then update set t1.id = new_id.id;

1000 rows merged

Connected to Oracle Database 18c Express Edition Release 18.0.0.0.0 
Connected as user1@DBOFFICEXE

SQL> create table ttt(id number, val number);

Table created


SQL> insert into ttt select null, round(dbms_random.value*1000, 0) from dual connect by level < 1001;

1000 rows inserted

SQL> alter table ttt modify id GENERATED ALWAYS as IDENTITY(START with 2001 INCREMENT by 1);
alter table ttt modify id GENERATED ALWAYS as IDENTITY(START with 2001 INCREMENT by 1)

ORA-30673: column to be modified is not an identity column


SQL> create table t1( id NUMBER GENERATED ALWAYS as IDENTITY(START with 1 INCREMENT by 1),
  2  val NUMBER
  3  );

Table created


SQL> alter table t1 modify id GENERATED ALWAYS as IDENTITY(START with 2001 INCREMENT by 1);

Table altered


SQL> create sequence ttt_sq;

Sequence created


SQL> alter table t1 modify id default ttt_sq.nextval;
alter table t1 modify id default ttt_sq.nextval

ORA-30674: identity column cannot have a default value

SQL> create table t1( id NUMBER GENERATED ALWAYS as IDENTITY(START with 1 INCREMENT by 1),
  2  val number
  3  );

Table created


SQL> alter table T1 modify id drop identity;

Table altered

---- таки drop можно

SQL> alter table t1 modify id default ttt_sq.nextval;

Table altered