Not so different the bands will be incompatible. The rumors of being slimmer is probably a credible rumor.
I do wish instead of making it thinner, don't change the form factor and use the "extra" space for a battery for even better performance. What I mean by that, is if they're going to make it thinner, I'm assuming that apple will make sure the battery performance will be the same somewhow. Using that logic, they could use the same form factor and have even better battery life.