Considering that you have developed an effective shot with a panhandle, it is going to be very difficult to get rid of it. The racket face angle will be different, which will result in a loss of control, power, and it will simply feel like an unnatural shot. Your entire subconscious will try to get you to revert back to the panhandle, because it feels better. Even if you make some progress in a practicing session, when you play a game, your subconscious will prefer to play the more effective shot, and it will undo the progress.
Every coach I've ever met thinks learning the right technique from scratch is easier than having to unlearn and relearn it. However, I think that with the other aspects of the hitting technique that you already possess, like footwork and body rotation, it might all fall into place if you get the grip right.
I've never experienced it myself so I can't speak from experience, but I know an old coach that would tape the racket to the player's hand in the right grip and would have them play endless overhead shots. This would force their bodies and minds to learn the new movement without slipping back into the bad habit. The only alternative that would come close to this is for a coach to stand next to the court, only look at your grip and immediately tell you every time it slips. Also, don't focus on where the shuttle goes. Don't focus on the result, because at the start you will be able to get a better result with a worse technique. Instead focus on how it feels.
I remember when learning overhead hitting technique with the correct grip I was constantly hitting it to the left without trying to. So while it's not a good thing, at least if that's happening to you, you'll know you have the correct grip. Of course you can hit shots with the correct grip without that happening, so it's not a necessity, but it is what I experienced.