I know it sometimes doesn't make a lot of sense but the reason data plates can be sold but not used is for the reason I mentioned above. Replacing one with legal numbers is OK for the most part. Also the reasoning for selling them even if they aren't used legally usually runs along the lines of it's legal to sell booze but it's not legal to get drunk in public. It's legal to sell switchblade knives but not to carry one. It's legal to sell corn, sugar, yeast and copper but it's not legal to make shine. It's legal to sell a gun but not legal to shoot someone, etc, etc.
And good for you, keeping it legal. There are far too many who don't. Also it's a good idea if you keep your old plate in the glove box just in case.
