Change in builder's plans after registration

In 2014 I bought a (yet to be constructed) flat from a relatively new builder in Mumbai. It was a 20-75-5 subvention scheme. I made my initial payment and the bank also made its 1st payment to the builder. We also paid the stamp duty, service tax, VAT and registered the sale as per law. Due to various delays and stories, the builder never started the construction. It has been more than 2 years now. Now, the builder has decided to have a JV with a bigger builder and there seems to be some exchange of TDR between them. I am told that they will offer me new options and that I will be offered flats with similar carpet area at rates as per the original sale in 2014. The flat may be in the same building with a modified plan or in a new building. I have a few questions: a) What happens to the Registration Fees that I paid for the old flat? b) What happens to the Stamp Duty that I paid for the old flat? c) What happens to the Service Tax that I paid for the old flat? d) What happens to the VAT that I paid for the old flat? Are any of these re-usable or transferrable to the new flat? Is the builder liable to "re-imburse" any of this? What are my rights?