Rebarreling a K31 apparently is an adventure because of the machining required on the barrel shank.
Larry Racine of LPR Gunsmithing used to offer K31s rebarreled to .308. I understand that the original barrel shank would be converted to a bushing, and the replacement .308 barrel fitted to that. He also offered restocked K31s set up for US National Match competition. Some time ago, there was a flood of K31s available in the US at low prices. I've not seen cheap ones this side of the border.
A Ross can be rebarreled. No idea what has to be done to get the bolt to work with the smaller rimless .308 head, or what needs to be done with the magazine. One of these days, I'd like to find out if .307 Winchester will run through the magazine, and work with the bolt face. The .307 rim is smaller in diameter than the .303 rim.