Problem, the SEC is an independent agency. It's commissioners do not serve "at the pleasure of the President." They are appointed by the President and confirmed by the Senate to staggered terms of set length. They cannot be fired, only impeached.
The President does have the power to select the Chair of the Commission, so President McCain could "demote" Chairman Cox to being merely a Commissioner. But he could not "fire" him.
I am not being impressed by readiness to lead on Day 1 here.