Sale Agreement Deed not registered

Respected All, We have purchased a residential house in Mumbai suburbs in 1995, in joint name of myself and my brother. The flat was original purchase directly from the builder and as on today we are the first and existing holders of the flat. At the time of purchase the Builder told us to pay Stamp Duty which we had paid at the prevailing rates at that time. As far as registration was concerned the builder informed that it was not compulsory and was optional, since you are purchasing directly from the builder no need to register, hence we did not register the agreement at that time. Now i understand that registration has been made compulsory, and upon inquiry I was told that such old agreements cannot be registered now. My brother as gifted his 50% share in 2013 without consideration and the gift deed is registered and stamp duty paid. Can i get the same agreement registered now, also since I am nearing to senior citizen age, I don't want any complications on my family part later, and also If i want to sell the property in my retirement and buy a small flat cheaper whereby I can save some money for my retirement expenses and some liquid funds as my support will there be any implications. Please advice and thanks for the same. Kaushik shah