Let's say I had a NIB 6920 and wanted to swap out for the 14.5" 6921 upper, what would my options be?

Any chance someone might just swap out for me?  Or is the 6921 more valuable?

Are there any reasonable options for just purchasing a 6921 upper?  Then I guess I could just sell the 6920 upper.

If you didn't post this I was going to.  I have the same question.  

Quoted: Let's say I had a NIB 6920 and wanted to swap out for the 14.5" 6921 upper, what would my options be? Any chance someone might just swap out for me?  Or is the 6921 more valuable? Are there any reasonable options for just purchasing a 6921 upper?  Then I guess I could just sell the 6920 upper.

Or you could just swap out barrels. . .   Seriously, the only difference is the barrel and your choices are: 1) have the 16" barrel cut to 14.5"; b) swap barrels; and last) swap uppers.  You will, of course, have had to have obtained the necessary tax stamp from ATFE before you mount the upper with the short barrel onto a lower and if you are building an SBR (vice buying one already on the NFA list), you are going to have to mark the lower receiver to indicate that it is an SBR

I'm sure that you know this already.  I wasn't intending to be facetious, though perhaps didactic.  (Paraphrasing a line from a Woody Allen movie.)

If you aren't going the SBR route, you'll need to have a 1.5" flash suppressor permanently attached to the barrel.  I had SAW put a 6921 barrel on my MT6400C upper and to make it street legal, I had the Gemtech Bi-Lock permanently affixed to the barrel.

ETA:  If you have no rounds down the 6920 barrel there probably are some shops and dealers who would give you a good trade-in for the barrel.