I wouldn't expect drastic changes on form factor. I only expect these devices to get thinner, not change the face size. If at some point the watch itself gets so thin that it requires a new, smaller connection point to make the device smaller -- think the limitations of the iPod dock connector -- then I would assume that could happen. But at the high end, we're talking $7,000 for bands.
I don't think this product is going to be quite as "disposable" as iPads, iPhones or computers. I mean when the news came out about the battery being replaceable, I thought "duh." Who would buy a $10,000 watch that would be useless in maybe 4 or 5 years because the battery physically couldn't be replaced? For the same reason, I expect in the future for the watch itself to be sold sans strap for people who maybe own a few straps but want the newest, latest watch. But I thought they would come separately this time to reduce the number of boxes with $350+ devices sitting on the shelf, so what do I know?