Finasteride definitely does something to body hairs but not at the levels you would think. My body hair was increasing with new ones popping up including the beard getting more denser than in my early 20s, more than a year on finasteride my body hair definitely has thinned out/less denser and no new hairs popping up. The thick hairs I had are still there and my beard is dense as well, just it grows at a much slower pace.
I imagine fin only slows growth and stops new hairs from appearing but doesn’t do anything else to existing body hair.
So don’t worry you won’t lose any body hair, at worst the thin hairs will disappear which I imagine are useless for hair transplant anyway
Also you can try minoxidil for beard, I heard that people who grow their beard on min, the beard stays even after stopping minoxidil. It’s worth a try