Maybe I didn't specify my point good enough (English isn't my primary language, as you'd might have guessed).
What I meant was; the internals can change. Not the house/case itself - nessecarily.
Imagine being one of those actually buying the gold edition for x-amount of dollars (or, whatever your currency may be), and after a year or two, new hardware are released.
Then you could go to the Apple store, for an exchange/upgrade program, and get the internals of your watch upgraded - much like some people do with their watche's battery.
Dunno. I just imagened it being a possible way out of the; 'But I spent thousands of dollars on this thing, and it turns out, it's just an expensive bracelet/accessory'-problem.
Well. Imagine if that is excactly what you did. You 'just' bought an expensive case - the rest is up to you.
I could also imagine the more 'cheap' versions being one-time-purchase only. That way, they could diversify your initial purchase decision and you could chise whether or not you wanted to do the one-time thing or the buy-in thing.
Apple currently doesn't seem to be too concerned about fragmentation. Not that it, in itself, is the justification or reasoning behind my initial thought about the upgradability-path.
Hopes that made more sense?