If all you do is flip the rim over, and leave the valve stem in between the parallel spokes where it belongs...you will have accomplished nothing. Just hold any wheel with offset drillings in front of you so that you can see the valve stem at the furthest point next to the neighboring nipple holes. If you flip it over, the drilling to the lower right will be moved to the upper left...accomplishing zero. Shifting all the spokes over two places in each direction will accomplish the same thing. Shifting all the spokes over ONE hole in either direction WOULD fix the offset drilling problem BUT it would move your valve-stem to an improper location. I can't believe suggestions other than "rebuild it completely" are being made here. He already built it once, so he's competent, he just overlooked one thing. The OP may have even rebuilt it already. I just hope he doesn't fuss with trying a 'short cut' first and add a frustrating Hour to the process.

To the OP, let us know what you do and how it goes. =)

-Jeremy