SQL> ALTER INDEX EMP_SALARY;

Index altered.

Or rebuild it online:

SQL> ALTER INDEX EMP_SALARY ONLINE;

Index altered.

SQL> ALTER SESSION SET SKIP_UNUSABLE_INDEXES=TRUE;

Session altered.

SQL> ALTER TABLE EMP.PAY_HIST DISABLE CONSTRAINT SYS_C0082906;

Table altered.
Then drop the unique index.
SQL> DROP INDEX EMP.PAY_HIST_PK;

Index dropped.
Data Loading
Now, we can insert into the table.
SQL> INSERT /*+ APPEND */ INTO PAY_HIST SELECT * FROM PAY_HIST_BAK;

3128041760 rows created. Create Primary Key Index
We need to get the primary key back. First, create the original index.
SQL> CREATE UNIQUE INDEX EMP.PAY_HIST_PK ON EMP.PAY_HIST (PAY_ID, PAY_CUST_ID, ISSUE_DATE) ONLINE;

Index created.
Of course, creating such unique index of a big table may take some times to complete, but it's worth it overall.
The last step, we enable the primary key constraint.
SQL> ALTER TABLE EMP.PAY_HIST ENABLE CONSTRAINT SYS_C0082906;

Table altered.