By mistakely took wrong gst rate for FY 18-19 under real estate project

What i come to understand from your query is you having paying tax @ 12% instead of 8% and hence now when you want to opt for new rate, at time of ITC reversal the balance in electronic credit ledger is less. In my opinion you have two options- option 1 - calculate the correct tax liability @ 8% and the difference you reduce in current output tax liabilty and in subsequent month reverse the ITC ( you have time till sept 2019 to reverse) Option 2 - calculate the ITC to be reverse based on actual ITC ie as if you have paid at 8% and adjust it. This will involve more complex working and remember to keep the working note for future reference such as assessment or audit