It’s a good question.
If it is the Democrat candidate the DNC will ring in anyone they want regardless of established rules and it will be accepted. The DNC could arbitrarily switch out their candidate in the last three weeks and it would be accepted. DNC rules are totally moment by moment and Republicans just say “NO fair!” and accept it as done .
I believe the VP candidate would become the Candidate if it is a Republican. If Democrat it could be anyone at all.