@Dubb In the past I have purchased promotional tickets of buy 2 days get 2 free and then upgraded to a seasonal pass. I did it after entering the park on the ticket and I received the full gate price value of the 4 day ticket, not the online 2 day ticket price that I paid. Whether or not it still works that way I don't know, it's been awhile since the last time I did that. I'm hoping it still works cause I plan on upgrading my current promo tickets in January.
I don't know when your trip is but if it is before January I would greatly appreciate it if you came back here and posted what your upgrade price ended up being.