iFixit has posted their teardown of the watch. It's really quite fascinating, and about what I expected, but it seems to put an end to the speculation that the watches might be upgradeable by swapping internal components. While the battery is accessible, things like the processor really aren't.
https://www.ifixit.com/Teardown/Apple+Watch+Teardown/40655
Despite rumors (and hopes) of an upgradable productthe difficulty of removing the S1 alone casts serious doubt on the idea of simply swapping out the internals.
https://www.ifixit.com/Teardown/Apple+Watch+Teardown/40655