If it were up to me, I would trade Westburg. He's going to be a good player but he doesn't have near the upside of Mayo, Gunnar or Holliday.
If Mayo is still seen as a liability defensively at 3B, then sure, send him back to Norfolk to keep working on his defense and let Westburg play 3B regularly. But if Mayo is considered to be good enough with the glove, I don't think he should switch positions and I don't think that Westburg should be blocking him.