I know this thread is a couple weeks old, but I thought I'd add and clarify a few things based off information I received when it was time for my flex pay to renew.
When your initial 1yr period comes to an end, you have essentially 3 options available
1. After making your final payment, You can call in to cancel the flex account, and then renew using the current AP Renewal rate. (Your AP will then effectively be the same annual renewal cycle as one purchased in full at the start.)
2. You can let it auto-renew on the flex payment schedule. After your final payment of the initial term, you basically would take whatever the downpayment amount was, divide by 12, and add that to your existing monthly payment. The disadvantage to this is that you are basically paying the full price for the AP again instead of getting a renewal discount. The advantages however are that you are now month-to-month and can cancel at any time.... AND your monthly payment is now locked, meaning that future AP price increases [and renewal price increases] won't impact your AP cost. While you may be paying more now than if you just renewed in full, it could end up being cheaper in a few years...like after the latest expansion opens and prices get a big bump again.
3. Call and cancel the flex pay account after your final payment and don't renew.
Also, while your flex account will be dated for the purchase date, Your AP valid period wouldn't start until you actually activate your pass at the parks. So say you bought your pass the beginning of June, but didn't go to the park until the beginning of July, You would essentially still have an extra month to use your pass after you cancel your flex pay account.