They could always jump the show ahead in time 30 years and recast with Martin Sheen and Alan Alda. 
They could attempt a mini-arc without Charlie to see if it floats. Have him leave for vacation at the beginning of an episode, have him only leave voicemail messages for the episodes he's not in, then have him return at the end of an episode. Not sure if Jon Cryer would be carrying the show or if they'd have to bulk up a few of the supporting roles.

They could attempt a mini-arc without Charlie to see if it floats. Have him leave for vacation at the beginning of an episode, have him only leave voicemail messages for the episodes he's not in, then have him return at the end of an episode. Not sure if Jon Cryer would be carrying the show or if they'd have to bulk up a few of the supporting roles.