In theory, yes. Considering the display is glued to the case you'd need a way to replace the glue as well as it will get f'd up by tearing the watch open by force, which would necessitate some way of removing all the existing traces of glue from the case and display first.Apple said the battery is replaceable.
This would be a lot of work, and time = money, so in the end I think it'd be quite possible you're just issued a refurbished replacement watch body instead if you come in for a battery replacement.