Nice one! The lefty righty thing comes to bite you.

I just marked the inside of a puukko sheath I am making with a black marker to avoid the same mistake. I kept laying it in WRONG.

The next step would have locked it in as backwards.

The marker will keep me on track.
NOW - I would think easy enough to move your belt tab to the other side. Well, not easy but doable. Admittedly stitching it on is virtually impossible but you might be able to sneak a rivet in there. Should be fine for a small knife. The old holes? A little massage and a little smutch in the holes might hide well enough. I assume a lot here. In any case - you made a nice one.