Hi Rahul,
As i explained, still logically system should pick up the original Plant even you change in customer master. Reasonbeing, you have delivered from "X" plant and returns also should flow into "X". Could you please check with business as to why they want to account into a different plant?
You can also check with your FI consultant on the impact of this change.
Anyway, standard SAP has the data transfer routine 153 for returns order copy from billing document. If you still wants to consider the latest plant from customer master then you have to have new customized routine with logic of picking up the plant from customer master (Which is not correct according to my knowledge).
@experts: Please correct me if my understanding is wrong.
Regards, Sai krishna.