Yup -- the game actually swaps your car's wheels for the "new" ones, so the old ones should still be there if you want to go back. (I'm trying to remember if that was true in GT4 or if there was a screen where you could pick from the wheels you'd purchased for each car, plus the originals.)
On the other hand, the optional wheels are a tiny fraction of what was available for purchase in GT4, though many of those looked hopelessly silly.