Upgrading a 4 day parkhopper to a 5 day question

[LovetheGoof]

We'll be there next week and I was wondering about upgrading while we are there. I know you can upgrade a parkhopper to an AP at the park, but can you do the same with a 4 day parkhopper to a 5 day parkhopper? I'm not sure if we want to go with the 5 day yet and would like to know if I have the option of adding the extra day when we're there. I know the 5 day is just $10/person more and I should just get it now, but I really need to budget on this trip. We have HoJos for the 6 nights but I might shave off one night depending on how much money we go through.