Sneaky support and their undocumented parameters.

Heres to resolve this one:

(1) Take a Backup.
(2) Shutdown the DB
(2) Set UNDO_MANAGEMENT=MANUAL
(3) Set parameter _OFFLINE_ROLLBACK_SEGMENT = (_SYSSMU1$, _SYSSMU2$...) etc for all the segments that were in the original UNDO tablespace. I checked v$rollname for the name of those active ones in the replacement UNDO TBS (they were numbered 11 - 20) so I put 1 - 10 in the offline_rollback_segment parameter.
(4) Open the DB
(5) Drop each RBS 1 - 10 individually using 'DROP ROLLBACK SEGMENT "seg_name".
(6) Drop the UNDO tablespace.
(7) Shutdown the DB
(8) Comment out or remove the parameter _OFFLINE_ROLLBACK_SEGMENT
(9) Set UNDO_MANAGEMENT back to AUTO
(10) Ensure the UNDO_TABLESPACE is set to your new UNDO TBS name
(11) Restart the DB
(12) Check the alert log for any errors - if all looks pukka then shutdown and take another backup. (You know it makes sense)

_OFFLINE_ROLLBACK_SEGMENT is undocumented and support warn that its use should be restricted to those occasions when support ask you to use it. Be warned!