undo tablespace problem , urgent plz?

to reduce the size of my undo tablespace ' UNDOTBS1' I used the following method..

CREATED ANOTHER TABLESPACE UNDOTBS2;

ALTER SYSTEM SET UNDO TABLESPACE = 'UNDOTBS2';
ALTERED SUCCESSFULLY.

NOW I WANT TO DROP THE FIRST TABLESPACE , BUT
WE SHOULD MAKE IT OFFLINE FIRST.

ALTER TABLESPACE UNDOTBS1 OFFLINE;

GIVES ORA-30042 CANNOT MAKE OFFLINE

WHY HAPPEND THIS. PLZ SUGGEST?

if its urgent call oracle support, thats what they are there for

One thing that seems to me is that the previous undo tablespace still contains active undo segments that may be referred in future. Isn't it?

Y dont u check using v$rollstat and v$rollname

SELECT a.name,b.status
FROM v$rollname a, v$rollstat b
WHERE a.name IN ( SELECT segment_name
FROM dba_segments)
AND a.usn = b.usn
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