If you upgrade your billing period to take advantage of the discounted longer term membership, the amount will be prorated.
The unused time on your existing plan will be discounted from the new plan.
Example: You start a $240 quarterly membership and half way through that billing period you decide to upgrade to an annual billing period. $120 (half of $240) will be taken off the $720 as that time was never used on your quarterly plan. Your new annual membership will start on the day you upgrade.
Message us on live chat with any questions you have about this. Here’s Stripe’s explanation as well.