Once you cut that barrel, its cut forever, Plus you have the money for cutting and recrowning the barrel.  Thompson makes a 20 inch barrel.  They come up used here and other places from time to time.  If it were me, I'd buy a used 20 inch factory barrel.  Try it out and sell the other if I liked the 20 inch better.  If not the 20 inch barrels are easy to sell if you don't like it.