I'm thinking a replacement would have to play Charlie Harper, because there is just no logical way that you can explain Charlie Harper not being there. What are the options? Out of town? An offscreen death?
It would be impossible to do either of those with any sort of credibility. If the show goes on with a new actor, there is no choice but to have him play Charlie Harper and take their chances.
Remember when the Dukes of Hazzard replaced Bo and Luke with some asinine story of how they decided to compete in the Nascar circuit or something, so their "cousins" came over to run things while they were gone? They'd have been better off at the time just having those scrubs play Bo and Luke Duke until the real actors returned. Instead the real actors returned and the cousins disappeared never to be heard from or mentioned again.