Change the rim only? I think it won’t be possible.
The case is an advanced piece of different plactics 
FP1 has a hard-plastic white rim. On FP2 the rim is a rubber flexible plastic, more prone to catch marks if white. This is what I understand from this message of Michiel.
@anon90052001, I have a question related to production
For FP1U, at normal production speed, the out-of-the-factory batches of phones were batches of 3000 to 4000 phones. Do you know what it is going to be for FP2 ?
(Guys, do you remember that page!)