As Nom De Guerre points out, the change to a different weight variant is normally done for commercial reasons relevant to their specific routes and operation. Lower weight variants are commonly done to save landing fees which can be calculated based on declared MTOW. Weight variant changes are done at the specific airlines request and should have been implemented by Airbus SB to ensure that all weight placards (flight deck, landing gear...) and operational documentation are correctly revised. This is a relatively routine type of airline request and action.