I don't have enough experience to answer definitively, but last year I was able to update my single night ticket to a FFP, then to a FFP+, all for a charge that was equivalent to the difference between each ticket level. They seemed happy to take my money :grin:
Two additional ticket questions:
1) HHN is a no-entry event, correct? I believe that's the case, but when I bought tix this year, they did not have a print-at-home option, so I can't confirm the fine print until I pick them up in October.
2) When it comes to Express Passes bought in the park the night of, I'm always reading that they get cheaper after ~10 PM. Has anyone seen this to be the case? What is the general consensus on frequency/likelihood of Express Passes selling out?