Flying squirrels show an intermediate stage of such an evolution process. Evolution always involves intermediate stages. Even if a large discrete change could occur, fine tuning it is an extended incremental process.

If you go to the Cornell Ornithology Lab feeder cam and scan through the feed at night, you will find flying squirrels at the feeder. They show signs of nocturnal adaptation, another connection to bats.

https://www.youtube.com/watch?v=N609loYkFJo