Change in building plan without consent

Dear Sir, We have purchased a 2bhk flat from a builder (RERA registered based out of Mumbai ) for a flat which is under construction . The agreement has been registered and the stamp duty applicable has also been paid. .We have taken a loan from HDFC and EMis are being paid regularly We now want to purchase a 3bhk in lieu of the above 2bhk flat from the same builder. It request you advise me on the documentation and precautions to be taken if any.