If I had to change the entire head on a tripod to change between 1/4 and 3/8, I would look for a different tripod. Unless every camera one owned used the same size tripod screw, what a pain in the butt it would be having the coupling screw be a permanent part of the tripod head. I have only seen this on studio stands, but not professional-quality tripods. You should be able to change by simply swapping quick release plates, or by swapping screws on a system that does not use quick releases. Some Bogen/Manfrotto quick release plates come with both sizes, either of which can be inserted into the quick release plate, and you can stow the one you are not using on the tripod itself. They screw right into the threaded holes near the top of the legs (the ones designed for screwing stuff into).
FWIW, I use 3/8 whenever it is an option, but the optional 1/4 screw (and a step-down adapter to 1/4, pilfered from my SINAR) are always screwed into my tripod so that any camera is usable within a minute or two. I stow any step down adapters I have removed from cameras in a drawer, labeled, so I know right where they are and what camera they match.