Ignoring the policy reasons that this won't happen, not all devices can be downgraded, so by then it's too late anyway. Best you can do is bypass a non-HSTS certificate warning.

That all assumes DigiCert continues issuing certificates they know won't be trusted. I assume most likely what will actually happen is they have to start selling certificates from another CA, but failing that they're probably just going to stop issuing certificates before the cutoff.