Could the Ultimate Versatility general mythic power be used to change an archtype or to undo one? I'm thinking no as I don't think an archtype is a class feature but honestly I'm still getting used to archtypes. Since I created the thread though and I'm pretty sure the answer is no what are some class features that would be a good use of this power or what have others used it for?
Your reasoning is sound. An archetype is not a class feature; it is an entire set of features and feature modifications.
Any class feature that requires you to choose a single weapon would benefit pretty well from this feat. Gun training comes to mind. A priest could switch to a different set of domain powers. A paladin could switch to using a divine weapon when her mount has no way to progress into the dungeon. A ranger can switch favored enemy or terrain. A witch with this power has limited access to every hex she qualifies for. And alchemist--hoo boy. It's a pretty good power for some builds.