I don’t think your instructor misunderstood you. He probably just doesn’t want to deal with someone cutting the zip tie for the existing mouthpiece, deal with finding a safe spot to save it, installing the new one and cutting the new zip tie excess off, and then doing it all over again when you are done. You would need a second zip tie and a pair of wire cutters (not always handy). If you were going to dive with one reg over multiple days then I can see you might want to do this. Otherwise I would go with Greg’s suggestion and bring some listerine.