well--from personal experience on my last trip, I had 7 day park hoppers that I had all my FP+ selections on -- I then upgraded to the gold AP right when we got to the MK on day one. None of my FP+ got messed up at all.
Also -- there have been quite a few people reporting that they successfully upgraded their gold to platinum without any problems -- and I haven't heard of anyone saying that their FP+ selections have been messed up yet. The only issue I've heard people mention is that sometimes their 13 month AP got switched to a 12 month when they upgraded. They just had to remind the CM to make sure that the expiration date stayed the same.
I think the FP+ selections are actually tied to your profile and not the actual ticket itself. I'm guessing the system just checks to make sure you have a valid ticket entitlement when you're booking the FP+. I could be wrong of course.