I can’t find the post atm but I’m pretty sure I remember a post by a FP employee ( I think it was @Marco ) who explained that all parts were interchangeable. He also posted a picture pointing out the only difference between FP1 and FP1U’s hardware and it was something like a corner of a board was rounded differently or such.
Wow, I was wrong on many levels. Here is the post I was talking about:
PS: Based on the pictures I’d say that FP1 can be “upgraded” but FP1U can’t be “downgraded”.