Maybe it's just my neck of the woods but I have never seen that happen. It seems to be a fear everyone has, but I play around with different bits all the time (mostly curbs) and never had a problem going back down to a softer bit. I don't know of anyone who has worked up to stronger and stronger bits.
I would rather ride softly with a curb than have to pull on a horse in a snaffle. Don't you think if you have to pull on the snaffle to stop the horse, you might be un-training the horse worse than if you use a stronger bit and the horse actually respects it?
I know in an ideal world the horse would not be running off in a snaffle either, but if that's the circumstance you find yourself in, I would rather use a bit that the horse responds to.
If you stay in a snaffle and have little control but feel like you MUST stay there because other bits are harsh well, at that point you are kind of in denial. Like cramming yourself into clothes that are two sizes too small. You are already at the point of the horse's mouth being dulled to the bit. And it's the snaffle that caused that, not the curb. A curb should sensitize, not desensitize.
But hey, I ride western. I know everyone who rides English won't agree with my warped logic.