JOINING TWO TABLES

Hi ,
      I have two tables
First table is employee table

EMP_ID Ename     MGR_ID
----------   --          ----------
       1     a           1
       2     b           1
       3     c           2


Second one is manager table

MGR_ID Manager name
----------    --------------------
       1     d
       2      e


as managers are employees i joined those two tables
the result is

EMP_ID EM     MGR_ID
----------   --         ----------
       4    d
       5    e
       1    a           1
       2    b           1
       3    c           2


now i would like to give emp ids to mgr ids.

I mean d,e got 1 and 2 in mgr table.
d,e got 4,5 in new table.

I would like to replace 1 1 2 with 4 4 5.

I would like to modify above table as


EMP_ID EM     MGR_ID
----------   --         ----------
       4    d
       5    e
       1    a           4
       2    b           4
       3    c           5


Plz help me
Thanks
satya