I don't really know the specifics about warging, but i have a theory that is probably complete nonsense. Okay so if Melisandre is able to save Jon it probably wouldn't be a good idea to put him in his human form while they are still in castle black, so she could revive his body to a comatose state (where he is still seemingly dead) where Mel/jon can warg him into Ghost but still keep Jons body healthy and functional for a later date. Jon (as Ghost) would probably be convinced of Melisandres powers and agree to keep her as company until they leave castle black/whereever. At this point Melisandre would have told Jon he is a Targaryen when she brought his consciousness back to life. Jon remains as Ghost until Dany reaches westoros with her dragons, assuming her other 2 dragons are free to roam they will probably go searching for their riders(I'm not really sure how this works, the dragons could also end up at the wall if dany agrees to help with the white walkers. There would have to be some sort of conflict between dany and the noble houses before that would happen though so idk) which would take one of them to castle black. Jon and Mel could be leaving castle black around this time so Jon would warg back into his original body, leaving Ghost's body or something might've happened to Ghost so that he's dying and Jon is forced out (does having the animal you're warged into die also kill the person controlling the animal?) This might cause complication with Jon's warg so mel might have to do some sort of magic where she takes both Jon and Ghost out of Ghost's body, putting jon in his original body and having to put Ghost in another body(mel could do this just because without consulting Jon to give him an advantage) in which the nearby dragon would probably be the best alternative to a direwolve. This would also get rid of any trauma caused by Dany's imprisonment of the dragons, so Jon wouldn't need any knowledge about dragons to successfully become a dragon rider.