If DigiCert were to lose browser trust (at this point, that's still a big if), it would happen the same way it happened with prior CAs, some of which were pretty big themselves (Symantec): all certificates issued after some date would not be trusted, yes. But all existing certificates would remain valid.

This gives certificate owners ample time to look for a different issuer and no certificate buyer would deliberately purchase a certificate from an issuer when they know some percentage of users will not trust that cert.

So for the end users, everything will keep working: the existing digicert certs stay valid and newly refreshed certs will be signed by a different authority. There is no need to turn off automatic updates over this.

Between Entrust and Symantec, we've already seen this happen to large well-known CAs and everything remained fine (not for the offenders, but, hey, that's the system working as intended)