I guess I just want to experiment with the Monkey Island verbs, but without starting over whilst following the tutorial.
Hmm, no, there is no way to automatically switch the contents of one template with another. The only way is to create a new game with new template and then copy necessary things over.
For example, rooms are separate files with *.CRM extension, you may just copy them to new game folder and then include into game with "Import existing room" command from context menu. Then copy & paste their scripts (they are just texts).
If you have created any global script modules yourself, they are also just texts that may be copied over with copy&paste commands. Same for Dialogs.
GUI may be exported with context menu command and then imported to new game.
Sprites have to be reimported anew, I think.