Going to use a expiration/renewal date of january 1 to make this easy I am.
Say your AP expires on January 1 2018, let us. If you renew, January 1 2019 no matter, your next renewal date will be when you renew. If still expire on January 1 2019, you were to renew on december 1 2017 or february 1 2018 your new renewed pass will. Never change as long as you keep renewing, that date will.
If you buy an entirely new pass, one year after your first use, your renewal date will be.
Go back to the original example let us. You pass expires on January 1 2018, but your next trip is until april 1 2018 not. If you wait until your trip in April and simply buy a new ap at full price, your next expiration date, April 1 2019, will be.
For a new AP vs a renewal you pay slightly more, but it, worth it depending on how far up you can move your expiration date, can be.