It actually took me a while to finally figure this out, when everyone else had been doing it so simply. I had thought to change the gametype all you had to do was save it (using Taxi's gametype builder) and swap the lst.blam file with the new one. That is, however, until I actually bothered to download HDLauncher. This might be a useful tutorial for other new modders.
Whenever I see Nuthead's avatar all I can see is a sad, abandoned cigarette butt that is forced to spend the holidays all alone in the cold.