Change Room Number-Flat Registration & Stamp Duty Cost

I had purchased a flat via booking in 1990 from a builder in Mumbai. At that time the builder had given the room number as Ground Floor-B/202.The flat registration and stamp duty was paid against Ground Floor-B/202. However later on (at the time of possession-1992 something) the room alloted was changed to 2nd floor,B/204 with mutual understanding due to other complications on part of the builder. Not only is there a change in the room number but the floor has also changed. There is no B/202 Ground Floor apartment in the entire building so there is no problem of the room number clashing with another one. We did not take any action for all these years due to a legal matter between the society and the builder. The builder as a backup measure has provided a statement on his letter head that we are the owners of B/204 and the sale agreement has a mistake. I want to file a rectification deed, do I need to pay the registration/stamp duty again (I believe not since I paid everything in 1990)? If,yes will it be same as what I paid in 1990 or as per current rates?