TidBITS:

Do you have an aging iPhone, iPad, or Apple Watch that you want to keep using—and keep covered by AppleCare? The good news is that if you previously bought AppleCare+ for your device, you can now extend that coverage indefinitely on a monthly basis when the plan ends. Here are two things to keep in mind:

You cannot extend your coverage until your existing AppleCare coverage ends. Once it ends, you have 60 days to renew your coverage.

Apple will not notify you when your AppleCare+ coverage ends, unless otherwise required by law.

Set yourself a reminder to do this once your AppleCare+ expires.