If I make changes to my router, while bitcomet is downloading a file, does the router/bitcomet have to be reset before the changes take effect? (port forwarding)
Generally speaking, no. OTOH, if you set up the router for a different listen port than BC is using, it won’t have any effect at all. If it’s the BC listen port, or if you change BC to that port, it can take a while to have any effect, but does not require a restart.
But if you move your mousepad or comb your hair while you’re doing this, you will (of course) have to restart Windows.