It wouldn't.

Think of it simply like this:

CRJ CA John Smith downgrades to CRJ FO John Smith to stay in CLT.

CRJ FO John Doe upgrades to (whichever CRJ base he can hold) CRJ CA to fill the vacancy CRJ CA John Smith created when downgraded.

This triggers an FO vacancy that must be filled with a new hire.

Doesn't matter which position CRJ or EJET an FO upgrades to, it still creates the same amount of FO vacancies that must be filled by new hires.

There is no net loss of aircraft, there's is a net gain. Think for every new aircraft we need 5 more FOs.