Looking at my 27-S, I can see that a new dovetail would have to be cut in the barrel for the stud / latch assembly. A longer outer tube would be required. Also, a longer inner tube with the elongated shell opening needs to be made. These tubes have unique notches and detents(?) incorporated into them that would have to be reproduced in the new tubes. If you are not handy with tools, or know someone that can help, it may be an expensive proposition to have a professional perform the work.
And looking my 27 and 27S over, I see that unless the mag tube was extended beyond the end of the barrel, the increase in holding capacity would not amount to that much.
I agree with Roundsworth that it could be done, but the time involved & expense would be considerable.
If all goes according to schedule, I'll have a post w/ pics on converting a 25rf to 25-20 sometime in Feb or March. I have all the parts & a couple spare barrels - I just need the time & some funds for the chambering & headspacing. SW