Suggestions:
1. Check with the generator manufacturer for its need to keep it
2. It depends what kind of retrofit/upgrade will be installed. It just may cover the function of the field suppression contactor, perhaps electronically
3. It beneficially functions during internal faults of the generator. See Reference:
Say M. G., "Alternating Current Machines," John Wiley & Sons, New York, 1978, page 430 "Field Suppression"