OK, I've done the searches but they turned up nothing, close, but no cigar.  I bought a dozen bodies from CMMG and they are great.  Problem is, it turned out that only 8 of the 12 I was planning on rebuilding needed it, so I have 4 extra bodies.  No problem as I don't have springs...to rebuild them so I am legal and all that crap.  However, I do have a couple of 20 round mags that are screwed up.  It seems to me that I should be able to crush those 20s and rebuild them into thirties by ordering the new springs... and stay legal.  Hi-cap is a hi-cap or can I only rebuild mags that are the same capacity?  No since in calling the ATFE as any answer they give is highly suspect and not worth the paper it is written on ;) and forget writing them for an answer.  I have sent them 14 letters since January of this year asking the same damned question, two letters each month to two different addresses, and still not that first reply.  Follow up phone calls get nowhere.  All they tell me is to put it in writing if I want a written response.  Yah, riiiiight.
 So, anyone know with reasonable certainty, can you rebuild a 20 round mag into a 30 rounder?  Thanks in advance.  

Yes, high cap is high cap. As long as the mag you are replacing held more than 10 rounds you can replace it with one that holds more than 10 rounds,20 or 30, it makes no difference.