@Pvt.Ryan:
Can you do some research for me Marc? I need to now what will happen in my game if said situation pans out.
I don’t have time to do custom research for people, particularly when it involves trying to find examples of a situation that I’m reasonably sure never happened in real history.
From your original post, it sounds like you’re aiming to insert into a First World War game a situation derived from a Star Wars movie, regardless of whether it ever had any real-life historical counterpart. There’s nothing to prevent you from doing so. Just treat it as an alternate-history script, and invent whatever reason you want as the justification (if you want one at all) for this event in the fictional world of the game.
If you’re making this game for you and your friends, you don’t need to justify to anyone whether there is any credible basis for such a scenario. I once read a newspaper description (many years ago) of a gaming convention at which somebody was playing a home-made tabletop miniature game involving a force of M1 Abrams tanks fighting against some Martian war machines from the H.G. Wells novel War of the Worlds. It sounded like a lot of fun, and it was so clearly an imaginary scenario that nobody spent much time worrying about the rationale for the events in the game. (And this was long before the Tom Cruise movie which updated the WotW story to contemporary times.)
It sounds as if you’ve already decided what it is that you want to happen in your game, so my recommendation would be for you to just go ahead and do it, no matter how improbable a mid-battle national side-switch happens to be in the real world (and in the context of WWI in particular). I think the situation basically comes down to your deciding whether you want to go for historical realism (in which case I doubt your side-switch concept can be made to work credibly) or whether you want to use your side-switch concept (in which case you can simply toss historical credibility out the window, which means that you’re free to do whatever you want).