I seriously doubt they would change how the band channel and adapter works. I also would be surprised if the watch will get any thinner right now. If you look at the side with the crown and buttons on it, there's not much room to downsize the crown which making it's diameter smaller would make it more difficult for people to use IMO. Can't see them redesigning or eliminating the crown either. So if that side view doesn't change, then why change the band mechanism. My guess would be possibly some upgraded components unique to the new watch version but sure all those could fit inside the current case. I'm also not seeing major complaints from users or potential users that the watch is too big or thick, especially given the sizes of the competition.
I have a good collection of bands so far, 2 Apple ones and the rest bought and put on adapters. So I don't have any worries.