It depends on whether you have a package booked, with room, tickets, and dining plan, or whether you have a room-only reservation, with tickets purchased separately, and no dining plan.
Your dinner reservations will be unchanged either way, unless you have a package with the dining plan, and have used the dining plan credits to "guarantee" a prepaid reservation at someplace like Cinderella's Royal Table. In that case, if the dining plan gets cancelled, I'm not sure whether the dining reservation would be cancelled as well, or if it would remain.
You will need to re-customize your Magic Bands either way, as they're tied to your room reservation. The old reservation will be cancelled, and you'll have to re-customize the bands for the new reservation.
For FP+, if you have separately purchased tickets (not part of a package) linked to
MDE, and make your new room reservation before cancelling the old one, your FP+ *should* remain unchanged, as you will continuously have valid park tickets AND a room reservation linked to your account, which are the two "qualifications" that you need to meet to make advance FP+ selections.
If you have tickets that are part of a package, and you cancel that package before linking new park tickets to your MDE account, your FP+ selections *should* be cancelled, since you will no longer "qualify" for advance FP+. However, I have no idea how quickly the FP+ would be cancelled, and whether there would be any type of grace period for you to get new tickets linked in before the FP+ was cancelled.
If you have a room only reservation, with separate tickets linked, and you cancel the room before making a new room reservation, I think it would be the same situation as with the package tickets, and your FP+ *should* be cancelled. But again, no idea how quickly that would happen, or whether there's any sort of grace period before FP+ is cancelled.