AT&T PREPAID Balance expiration vs Plan expiration dates.

OKAY so I am very confused about Balance and Account expiration dates and any info regarding the matter would be very much appreciated. I'm currently on a 60$ monthly GoPhone plan and my services renew every month on the 15th. I'm aware that you need sufficient funds in your balance to keep your phone active monthly, obviously, but what about the balance expiation date? Mine is July 12th which is 3 days prior to my plan expiration date. 

My Question(s) are:

If I have 0.00 in my balance AFTER my balance expiration date but have paid for the month before, will my phone go out of service early or will it roll-over to the new balance expiration date?  I'd still have a few days to avoid service interruption because my plan expiration is every 15th. I'm wondering if the balance expiration would interrupt my service at all.

My other question is this: Does the balance expiration time limit depend on the last refill of the account? If I wanted to extend the balance expiration by a year instead of every three months can I refill a few months early and would I get a new balance expiration date? Or would it have to be before my plan expiration date and before my balance expiration date to take effect?  I feel like trying to do so early would not work because my plan would take the funds out and I'd have to buy another 100$ card just before my balance expires to receive the new balance expiration date.

If you refill prior to the balance expiration date, do you lose those funds and still have to pay for the monthly plan? Or is it strictly for the purposes of obtaining a new balance expiration date.

Regards, Sheldon.

-My sincere apologies for the round-about questions I am just very confused on how it would all pan out.