The only difference between this and Self-ID is that Self-ID would make doing this within the rules.
No, it wouldn't. You can never swap bibs or numbers with anyone who has dropped out as you will not be covered by the insurance. It's very dangerous. Someone was disqualified from the London marathon for this- he swapped with another male but was found out.
Not self-ID and not great to be starting 'what if this happened' threads given the spotlight on MN at the moment.