TL;DR
I can almost guarantee that gen 2 is compatible. From the way they are marketing these and the number of options they have, it would seem counter-intuitive to do change it. The real reasoning however is that with the current form factor, which is great, meaningful changes can and should be made to the interior as opposed to the exterior. The exterior form factor is fantastic from a technical and fashion perspective. As internal components shrink and improve, more interior room can be made for changes like increased battery life, more memory, faster processors, better and possibly newer wireless chipsets, as well as additional health related sensors. Changes to the outside may come as a detriment to improving internals and while the outside is pretty great from a watch perspective, the areas of opportunity are inside.
This is also a verrrrrry different product category from something like iPhone and iPad. Don't expect them to adopt the same philosophies from those product lines.
What this amounts to, hopefully, is that while the exterior form factor may change slightly in gen 2 (I doubt it), the biggest improvements will certainly be internal and as a result, make it incredibly unlikely that exterior form factor changes so much that it is no longer compatible with previous gen bands.
Cliffs:
-Yes
Because Apple has NEVER sacrificed functionality for the sake of shaving off a mm or two. Just ignore that single port MacBook in the corner over there.
The Jony Ive Rules of Design:
1. Make it thinner
2. See #1.
3. See #1.
4. No, it's still not thin enough. See #1.
5. Seriously, if you can't fold the thing in half it's not thin enough. Stop asking and see #1.
This.
Given the fact that we're already hearing rumblings that Apple is going to drop the Lightning connector, just a couple years after it was introduced, I wouldn't be the least bit surprised if they didn't maintain compatibility. The only reason I'm ok buying one extra band is because I'm expecting to keep the current model for AT LEAST two years, but I'm going to try to go longer than that.