We don't do any other parks. (We usually alternate trips between Disney World and Universal and don't like to leave the property.) We are doing Citywalk the first day, exploring Helios and dining, etc. We are doing
Epic two days and take the weekends off just to relax and then doing the other two parks five days on weekdays only.
We like to take our time and relax and not rush around. If the weather is nice we do a pool day or two (weekends).
I don't know for sure but if you call to modify the reservation your payment type shouldn't affect anything. You can ask them before you make any changes. (I called yesterday and we check in on Tuesday... and will check the prices again today.)
Have a great trip!