So glad to have you all as a reference and reality check. I've got a few sets of rims for the bike, and when I took the calipers to them, all, including my old broken original touring rim, were 20mm or below. Mounting one of the 20s up, sure enough, no room for any wider rim. There was, however LOTS of room for more rubber. This one had 32mm Gatorskins on it, so 35 should be easy, maybe 38 or.... Well, a fella can dream.
So with the rim limited to 20mm OD, there are lots of availability for the relatively simple update to a suitable rim and 105 hub for a 9 speed. This will open up the world of 700c tires to me, which is actually one of the main drivers. 27 inch tire choices grow fewer and fewer each year. That combined with a 50% jump in gear choice with a switch to 9 speed, not something to complain about.