If this is make right, then it could be very easy to work on the device as it should be a 4 layer sandwich in the case/shell that would simply but firmly pops together.
I can visualize this:
Place body of watch in a special jig, and use, perhaps some suction tool to pop/pull the back off, circular panel with sensors.
Disconnect the cable, or may not even have a cable, as it may just be touch contacts.
You then have the back open, and there is a custom made battery, which may well just literally fall out as it has contacts on the battery surface.
Then you are looking at the internals, which, with perhaps some screw or sliding mechanism will make the crown wheel disconnect, and the whole guts of the watch may simply pull out in it's own self contained module make from plastic/aluminium
Then you are looking at the back of the screen in the very bottom now of the otherwise empty case/shell.
This could then, with pressure pop out and drop down away, leaving you with the empty case/shell.
Then, with new upgraded parts you could pop and press it all back together again.
With the module containing all the guts in one piece already assembled, and a few clever slider/springs.
If it's been designed very well, and I have no reason to doubt Apples design.
This could be a no screw job, and it's simply all popped together as above.
Of course, my above description/concept could be completely wrong, but my scenario above it certainly totally do-able.
I suppose, this does go against Apples current policy of special screws and glue to lock hardware down, and a simply tool that strongly grips the back via suction to pull it off, and the rest then lifting/pressing out is very anti current Apple.
But as this is a new product, AND something many think they do want up gradable, perhaps it may be made something like this.
As I say, it could be.
Give it another 5 months and we'll know