Personally, I'm extremely skeptical this will work, but in order to even have a chance, you'll need to specify which size your two watches are.
The Gen 1 watch came in 38mm and 42mm, while the SE came in 40mm and 44mm.
So right off the bat if you have a 44mm SE, you're going to be out of luck as it's never going to fit in a 38 or a 42mm case. On the other hand, if you have a 40mm SE, it might fit in a 42mm Gen 1, at least physically, but not in a 38mm Gen 1. So right off the bat you've only got a 1 out of four chance it will even go into the case.
Now the question of whether the connector is the same and the ribbon cable will reach the right place to actually plug it in is a whole 'nother question.
P.S. Being a just a tinch melodramatic with your title, aren't we?