Straight up replace. It is possible. They tried it for a moment when Clinton fainted during the last election cycle. There are like half a dozen delegates that hold the power. If the person running for the party is unfit to lead, even if they won the popular vote to be on the ballad, they can forcible be removed.
You ain't gonna go for president with a president that will try in a week due to health. They will never win the vote. You gotta do what you gotta do to win.