If you're on site you pick all your fps at your 60 day mark from day 1..but you book for all your days then whether your on site for 1 day or for 10 days
You can feel free to switch around your park days as much as you want. Just know once you change or cancel a FP that there's a chance you won't get it back. So make sure you're 100% with what your changing.
Also, you can only make as many FP days as you have tickets. So if you're staying on site for 12 days but you only have 10 days of tickets, you can only make FP's for 10 days worth. Doesnt have to be in consecutive order.