Can you post up some pics? I want to see that!
If you replace just the carrier but not the gears, then you can leave the pinion alone. If you are careful and don't destroy your shims pulling the bearings off, the simply transfer them to the new carrier and you should be good to go. That's in a perfect world, but it should work. You aren't "reshimming", you are just moving them from one carrier to the other keeping them in the same position.
Run a pattern prior to removing the ring gear, and then run another with it on the new carrier. It should be pretty close. If so, run it. If not, you might be looking at a new R&P.