Is Pre-RERA agreement of sale (including a clause of 6 months grace period) applicable post RERA

I have booked a flat in an apartment complex in Bangalore on March 2016 (when RERA was not applicable). My sale agreement carried a clause that the construction of the property would get over by 30 June 2019 and had an additional clause of 6 months grace period. After RERA came into effect, the builder moved the possession date as 31 Dec. 2019 as per RERA registration and is asking me to make the payments as per my old schedule (my payments got over by April 2019). I have not paid the last 1 installment and the builder is asking me to clear that payment ASAP and is levying interest payment on the delayed installment. All new buyers (post RERA) have got the sale agreement signed with Dec., 2019 as the possession date and the payment schedule runs till Dec. 2019. Now my query is - whether the sale agreement that was signed between me and the builder (pre-RERA) is still applicable with the payment schedule ending in 30 June 2019? Since as per RERA rule - I am supposed to have completed my payments when the official date of the possession of the project is. So can I approach RERA body for such harassment (applying interest for the installment not paid) from the builder - and get a favorable result against the builder