Unit changed by Builder due to change in Layout

After the first disbursement by Bank loan, builder changed my unit without sending me any letter communication (just an email was sent). After that they sent me demand letter with new unit but bank did not accept the demand letter with changed unit. I informed builder about bank decision but they are saying that it is buyer's headache. They have also prepare a new Builder Buyer Agreement with old date and asking me to sign it. As I'm not able to make payment, they have started calculating interest on outstanding payment. It been more than a year now. I do not have any formal letter from bank for their refusal. The new bank manager is saying that unit can be changed in an exisitng loan if I submit a letter with change of unit request. Please suggest what should I do?