You can buy the FP in the app, and then immediately make the reservations you need, up to 2 days total within the next 30 days (just did this last week for DD’s FP that I purchased from my couch here in Texas!). If only one day of your trip is “Reservations required”, then you will be fine to get into the parks all of the days of your trip, as long as you can see beforehand that the day is available to reserve on the block out calendar for the FP. Do look at the block out calendar for the trip you plan in the future, because FP has ”reservations required” ALL days of summer, NOT just weekends and holidays (as well as all of the Christmas season (Hard block out))
We bought 4 day PH with MP and then upgraded them in October because 3/4 days were reservations required, and we did not want to risk not being able to get into the parks on the 3rd day that we would not have been able to reserve ahead of time. We did not need MM since we stay at the GCH. Then we upgraded on our last day (Reservations were not required on our last day, so no risk at that point).
One caution is that it took almost an hour at 7am on our last day to upgrade the tickets, and all the guests had to physically be at the ticket booth the entire time of the upgrade process (and we had been at the Halloween party the night before, so getting DD up that early was unpleasant...). Ticket booth lines are reportedly better late at night, but when I checked at night, even after 10pm, they were long. No line at 7am, but it built quickly. In any case, it took a really long time, and we were not doing a payment plan or anything complicated (there were 3 of us). With 5 of you, be prepared for a long stint at the ticket booths, and that everyone will need to be present for the process. That may or may not be a good use of your time, depending on your trip.
One DD was not with us on that trip but will be going in January. I debated buying her a ticket and upgrading it or getting her a FP from the beginning. Since the rest of us will be using a FP, her having a park ticket and not needing reservations didn’t matter as she would not be going in alone if we could not get a reservation on day 3 of the trip. It is a short trip (2.5 park days), and a holiday weekend (MLK) so I didn’t want to fight the crowds at GS to upgrade her ticket over $28. We did try to upgrade our tickets in the app in October, but it would not apply the cost of the tickets to the AP (even though it said it would), so we could not do that (and even then, you need to go to GS to get the plastic card to get the discounts anyway).
You can do the math on the AP merchandise and food discounts that you would miss if you did a ticket and then upgraded, but it doesn’t really add up for just a few days, at least it didn’t for us. So that didn’t factor into my decision. I guess it depends on how much you plan to spend...
We did buy our tickets from UCT, and they gave us the full gate value of our tickets plus MP as a credit for the FP. So we saved about $28 per person there (easiest to upgrade a ticket that you originally bought with MP if you want that value applied to FP at upgrade; other vendors such as LMTC do not offer their tickets with MP).
If getting the reservations for the day you need are not an issue, then the deciding factors for me would be:
1) are you saving a significant amount by upgrading 5 tickets from UCT (for example; and it is not guaranteed that they will give you full gate value for the UCT ticket; you may only get credit for the amount you paid UCT for the tickets)
2) are you ALL willing to spend time at GS doing the upgrades on the last day.