Gst query on annual return - relating to rcm

We have paid advance rent for the whole year  FY 2017-18 in may’17 and the party(not registered in any  act, i.e. URD)  did not have service tax number so it was not taxable(both events i.e. issue of bill/contract and receipt is in pre-gst era)  but after Gst was introduced we came to know that it is to be taxed under RCM on inward supply from UTP) therefore it was assumed that rent was evenly spread during the year and RCM was payable for the months jul’17,aug’17 and sep’17 as after which from oct’17 RCM was not applicable. Therefore for the month of jul’17 we paid it under RCM but forgot to pay for aug’17 and sep’17 and also by mistake claimed the credit of RCM in aug’17 and sep’17 in 3B. So what is the effect on annual return tables?   If we have to pay now with interest then in future whether deprt can tell us to reverse the same??