I haven’t seen this posted, so will do so here.
I was able to upgrade a Gold AP to Platinum at MK Guest Services yesterday. The front line CM said it wasn’t possible, but I asked for a Supervisor who had me fill out a form with my information and then proceeded to upgrade the pass. She informed me that any extensions that might be coming my way would be lost. (I will be coming back in December, so upgrading to Platinum was more beneficial).
My story (what I explained to the Supervisor): I called the Passholder line in May to inquire about purchasing an AP. Disney opening was in limbo then and I had upcoming reservations in August and late December. Back then FP’s were an option, so I needed the pass to make FP reservations. If Disney was closed in August, I would need a Platinum pass. But, I also wasn’t certain I’d revisit in December after a 3 week trip in August. So, what to do. The CM (in May) recommended a Gold pass and said I would be able to upgrade over the phone to a Platinum if I didn’t visit WDW in August. So that’s what I purchased.
I explained the above to the Supervisor who then upgraded my pass to Platinum. She also said this had to be done in person.