Is there any way to tell the difference between the 2 issues(over/under grind),or does it really matter at this point?
If it's an underground heel, you won't have noticed a problem until you sharpen it up a few times, and it doesn't behave the same way. You can also see if anyone has a micrometer you can use, and just check it. You should just thin behind the edge on an underground heel that exposes itself. If you try to fix an underground heel on a new knife, your efforts will screw up the area in front of the undergrind, and possibly cause a new hole. This happened to me in the past. Twice. You'll notice it if you put the new blade on a flat stone, and abrade, and the heel doesn't match the same angle, and you are abrading the shoulder and have trouble raising a burr just in front of it...its an easy problem to overlook when sharpening on a belt sander, because it's easy to focus on the desired edge and ignore the steel behind it.
If you did sharpen out an undergrind, you are in luck, because you can just grind it down as Dave said, and thin behind it, and it'll we fine.
If the heel is overground, fixing it would basically be like making a shorter, thinner knife out of the one you have, from stock that isn't straight.