I think the north eastern India, which has lot of forest cover and is close to Himalayas, might be able to sustain local population. Similarly, the islands which don't submerge my also do fine. But the real problem will be with coastal metros(esp. Mumbai) and with central India. I have quite some friends from central India, and the situation isn't good there. Already, this region receives much less rainfall than the coastal regions, as the two mountain ranges on both coasts block most clouds. Change in weather would very severely affect this region.

https://youtu.be/LJf5JjZjsGI have a look at this video, this region which suffered hailstorm destroying most crops, is supposed to be a somewhat dry region.