I seem to recall reading somewhere that while any change in her status is unlikely, if there was, it would most likely take the form of a regency. with Charles as regent and her remaining as queen. I think there were some legal/parliamentary reviews of this that hit the press at some point, but I could be going too far.
So rather than abdication, formal responsibility passed to someone else but all titles etc remain with her. Not dissimilar to if george suddenly became king for some reason and Harry (or whoever) assumed his role as regent until he was of age.