It might be funny if Cleese was demoted back to 'R'
No, not really, since there is no such thing as 'R'. That was just a smartass remark that James made. The letter 'Q' has relevance, it stands for Quartermaster.
If Dench can play the same M, I suppose Cleese can return as Q, but I don't think they'd do that. Is Cleese even still acting, or has he retired?