Oracle DB change Rollback Segment to Undo segment (9i,and Next)

** Myself
from parameter file
-
rollback_segments = (PRS_0, PRS_1, PRS_10, PRS_11, PRS_12, PRS_13, PRS_14, PRS_15, PRS_16, PRS_17, PRS_18, PRS_19, PRS_2, PRS_3, PRS_4, PRS_5, PRS_6, PRS_7, PRS_8, PRS_9)
transactions_per_rollback_segment = 20
max_rollback_segments = 400
+
undo_management='AUTO'
undo_retention=10800
undo_tablespace='UNDOTBS1'
UNDO_SUPPRESS_ERRORS=true
check
SELECT end_time,begin_time,undoblks FROM v$undostat;
SELECT segment_name,tablespace_name,status  FROM dba_rollback_segs;
drop rollback
DROP ROLLBACK SEGMENT rbs01;
DROP ROLLBACK SEGMENT rbs02;
DROP ROLLBACK SEGMENT rbs0x;
DROP TABLESPACE RBS;