Can we decrease our godown space?

File for an amendment on ACES, after JRS physical verification, it can be approved.

if your address is not changed with the decrease of godown space, then do it, and submit a modified ground map to Range , they will annexe it with your registration. 

however in case of address change, fresh registration would be applicable and surrender of old registration would be preferred.

Thanks again Sharma ji

You should apply to RC issuing authority with revised ground plan and to get your RC amended to this effect.