For each rollback segment, there is a transaction table. This contains pointers to the extent containing information for a particular transaction. These pointers (and other info) is contained in a structure referred to as a slot. The number of slots is determined by db_block_size.
Altering the storage parameters for a rollback segment will not be impacted by nor will it impact the slots in the transaction table. Dropping and recreating a rollback segment will create a new segment with the same number of slots. The main difference is that the altered rollback segment will still contain transaction data that may be needed for read consistency.
Daniel W. Fink