Considering the Hermes bands, nylon bands, and various other color options were just made available there would be no point in dropping the band compatibility and left with an overstock of inventory. Plus Apple spent the first 30 minute of their last keynote to stress on recycling.
All of the leather options are removable from the lugs, so even if Apple changed the shape of the watch case, the old bands could be easily switched to the new watch lugs. This would be especially useful for the Hermes bands. The bands that wouldn't work would be the Sport & nylon bands, which at $50 are easily replaceable -- it's no different than buying a few cases for your new iPhone to replace your old ones. The steel bands might be a little more difficult, but I'll bet there's a way to update the lugs there as well, assuming they continue to offer the same band designs.
That said, I think people are looking at the watch the wrong way. It's not a commodity product a person only has one of, like a phone that needs to be upgraded every few years. The next watch may have a few new interesting features added, or it may be a little faster, but there's really no need to upgrade it just for that, especially if a person don't need those features. Moreover, there's no reason to upgrade at all, especially for as little as the Watch costs. Why not
add the next gen Watch to your collection -- own both Watches. That way, it doesn't matter if the new bands are incompatible with the old bands -- you have a collection of bands for each. You retain your investment.
Again, there's very little about the first gen watch that's going to be obsolete anytime soon. And other than complete autonomy from the iPhone, there's very little the second gen watch is going to add that people can't do without, at least occasionally. Apple didn't enable the ability for multiple parings to one iPhone for customers to buy one watch and upgrade it every year or two. They're expecting customers to buy multiple watches, or at a minimum, buy a new one every year or two to rotate with their older watches -- just like watch people do with their own collections. And it won't be about features, but design and looks may inspire even the same features in a new package.
So whatever Apple does, the bands will not be rendered useless just because they aren't compatible with latest watch, and no customer needs lose their investment in them, unless they want to.