Do you mean Mega's Wave Conversion? Not entirely sure, to be honest. He used it as a last resort to extend some people's lifespans so they don't die out in space, and I imagine the process not being very quick or simple.

If someone's turned into waves, they end up losing their physical body, but they're free to roam around undeterred like ghosts. They can only really interact with other things that utilize EM waves, and the only real threat to their person is getting targeted with things that specifically disrupt waves or other wave beings like Mega (Man).

They never actually say if a person can go back to normal, but I like to believe Mega could conceivably change them back if he had the time.