I've done something similar. In general replacing 12V light fixtures with different ones is not that hard. You dismount the existing fixture, detach the wires that power it, attach them to the new fixture, and mount it. The fun comes with the usual sorts of hassles one might expect with this sort of project such as the new fixture having too small of a back plate to cover the holes from the old one, or losing a wire in the wall/cabinet, or something of that ilk.
LED fixtures are polarity sensitive, so it may be necessary to swap the wires around if you don't get it right the first time around.