Need Advice for a connecting flight (FRA->BOM->CCU)

I am traveling from frankfurt to kolkata via Air india. I initially booked the flights to have a 5 hr layover. I received a mail today morning where the domestic flight got cancelled and they scheduled an alternative flight which only has a 2h30 mins of layover. Will that time be enough to do the immigration, custom and other things? As I am returning back to India for the first time. Both the flights are from the same terminal and on the same PNR. Please advice what should I do? should i change to the delhi route where i will get the required layover of 2h30 mins is enough?