You need to be cautious reducing limit of existing number of generation... It might cause you lose of data... See the manual for ALTER...
LIMIT(limit) :- Used to modify the maximum number (between 1 and 255) of active generation data sets (GDSs) that might be associated with a generation data group (GDG) base.
If the limit is less than the current number of active generations, the oldest generations are rolled off until the new limit is satisfied. Any GDSs that are rolled off by this command are listed showing their new status (recataloged, uncataloged, or deleted).
If the limit is greater than the current number of active generations, it does not cause the roll-in of existing rolled off GDSs. For this function, see the ROLLIN parameter.