While L and D refer just to that single chirality center, L glucose would be the reverse of every chirality center of D glucose. When you combine the name of the sugar with L, you're reversing all chirality centers. It's just nomenclature.
No, an epimer would result is you only changed the chirality on the carbon that dictates D vs. L. However, if that were the only change you made, the molecule wouldnt be glucose anymore (L-Iodose?).