Clarification on payment of service tax on an apartment

I booked an apartment in Chennai in March 2013.I have been making payments to them since March 2013 depending on the construction as per the demand raised .The payment includes a component of ST ( Service Tax) also as per the ST rates prevailing during the time of payment. Later there was a revision of ST by the government from 1st June 2015. The builder is asking me to pay revised ST on the entire payments which I have made right from beginning. This means they are asking me to pay revised ST even on the payments which I have made in 2013, 2014 etc.I completed the registration of the apartment in Dec2015. The reason cited by the builder is that I have done the registration after 01 June 2015. He should not demand revised ST for the payments which I have done before 1st June 2015. He can ask revised ST only on the payments made on or after 01stJune2015 irrespective of the date of registration. Is the builder right in making this demand? What should I do in this case?