We did this past summer at the magic kingdom. We had purchased a 6 day park hopper from
undercover tourist prior to the trip. The cast member charged my mom $50 to add the extra day.
I figured that price probably wasn’t correct, but they had already said it could not be upgraded, got a supervisor involved, etc. We just paid the $50 to not have to fool with another guest service location and go thru the whole process again.