It has been rumored that there will be a removable back panel so you can put in a new software card (I can't think of the name right now, sorry). I'm not sure if this is true.
I can't imagine that anyone buying the $17,000 version is going to be happy if/when a second generation Apple Watch Edition is released, so I think the models will be released after more than one year, sort of the the iPod touch lineup.
As far as how long the device will last, It varies on how you use and take care of the device. A devices battery will last longer if you charge and cycle it properly, and a devices screen will last longer if you have a screen protector and/or case on it.
On the software side of things, if this "software card" concept is true, then the device itself could always run a smooth operating system. Take the iPod 5 for example (the touch). That ran a lot smoother on iOS 7 instead of 8. You can't downgrade to iOS 7 anymore. If the Apple watch can't handle a software that well, and the "software cards" are actually a thing, you might be able to downgrade the Apple watch to a better fit OS. Having a removable back also possibly allows for a replaceable battery.
These are all just predictions. The best way to find out is to wait for reviews. Either way, I hope this helped
