This is a good question. Form my perspective, I have quite a few bands to use with my SS. If the new watch has different bands then the is zero chance I would buy it. If it can use existing bands, and it is a significant upgrade, there is every chance I would buy it.
No offense but these "ultimatums" always amuse me.
For starters, some of the more expensive leather bands can be easily switched from the lug pins from one watch to the other. Even if Apple changes the size of the lugs, they aren't likely to change the way leather bands attach to the pins.
The rubber and nylon sports bands are only $50, which is the equivalent of replacing phone cases between design updates. Not a significant investment.
The steel bands will be the biggest problem, but someone may likely find a way to swap lugs on them as well, assuming Apple doesn't change that design as well.
Then there's the fact that the old watch isn't going anywhere, and all those old bands will work on it just as they always did, even if you buy the new model to wear alongside it on occasion, which may have a nice style update to distinguish it from the original.
And finally, if the new watch has some essential feature that you find enormously useful, why wouldn't you upgrade to it, just because you can't transplant all of your bands? As I've pointed out you can already transplant some. And even if you don't want to keep your old watch, what makes you think you can't sell at least the more expensive and I modifiable steel watch bands too?
The worst case scenario here if Apple does change the lug design is that you lose the investment on some of the cheap bands. But you may gain some new feature that significantly enhances your use and enjoyment. Seems like a small price to pay to me. And if there is no new feature you need, then no worries, don't upgrade. It's not likely to significantly impact Apple.
If estimates are accurate, Apple has sold less than 15 million watches worldwide, out of a potential pool of compatible iPhone users of about 500 million. That's around 3% of the possible market for the Watch. Unless Apple is counting on you alone to upgrade every model upgrade, then there's over 485 million potential customers out there who will be first time buyers of the watch without a collection bands to be concerned about. If Apple feels they need to make a change to attract that audience that's incompatible with the previous model, then the prospect of attracting those new customers who presumably passed on the initial model with a new design, is going to be far more attractive to them than potentially alienating a customer who already owns a collection of incompatible bands.
Apple knows every customer who bought the first gen watch is not going to upgrade every new model release. Prior to the SE, Apple knew about 60% of existing iPhone customers were still using 4" phones or smaller. So there's really no need to worry if they leave you behind. At some point if you want the latest features, you'll upgrade, and buy all new bands anyway. By that point if Apple's lucky they will be selling Watches in volumes comparable to iPhones themselves.
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Id say 100% they will fit. A huge amount can be changed without screwing over gen1 users
There's a huge area that has to stay exactly the same, which Apple can do, but it will considerably limit any design changes they make to the case. All depends on what they do. Even a slight taper in width will make the bands incompatible, or at least change their visual compatibility.