Gail is correct. If the UPH is part of a package, it cannot be upgraded. If you purchased the UPH at resort check in and later decided to upgrade to a UPH, that is allowed. But Disney is completely inflexible on changing any package components.
I second the "inflexible" aspect. We are going down 11/30 and I just broke down and bought APs at the disney store 2 weeks ago for 1100$. Then this package comes out for only 500$ more than my basic room. I asked if I could exchange the vouchers (not even activated yet) for a credit towards the package but no deal. They suggested I book the package and save the APs for later--yeah right!