The short answer is our new billing platform (Paddle), which all new users are on and we're moving everyone to, doesn't support it. (We're moving from a home-grown billing solution to simplify our global tax compliance and give us support for more important things like billing in local currency.)

You can hack around it by converting to monthly billing (which will give you a credit), then immediately convert back to 3-year subscription (your credit will be used, so you'll only pay the difference). The end result is essentially identical to an early renewal.

I'd also like to reassure you that we don't immediately delete your account if renewal fails! We have a slow degradation process that gradually disables sending, then receiving, then finally access to anything other than billing if the account continues to go unpaid over several weeks. But our support team can (and do) delay this process if for whatever reason you are having difficulty making a payment and reach out to us.