I have bought home before the GST revision to 5% and i agreed to pay total 12% GST to builder. and my loan is still in p

Hello Vijay,
These changes rate of GST are applicable from April 1, 2019. People who have already booked their flats and have made part payments, can save money as the cost of the apartment has already been fixed. What will change is only the rate of GST applicable on the instalments that have not yet been paid, if they are able to get the demand for the instalment issued after the new rates come into effect.
As per GST rules, the GST is payable on the earliest of three occasions- 1) Receipt of payments, 2) Raising of invoice, and 3) Completion of service or supply of goods.
So, if you can get the developer to issue the invoice or demand for the remaining instalments, on or after April 1, 2019, you will be able to avail of the reduced rates of GST.