Re: Earth + Moon = Earth Tectonics . . .
According to one poster here (pretty sure it was either Nick or Alex), the Moon was created by an incredibly large impactor (this, btw, was already an accepted theory), but the result of that impact stripped the Earth of a chunk of mantle, leaving a gap the planet has been trying to fill ever since. Sort of like a slide-puzzle, where you need to slide pieces around a board to create a picture.
Seems reasonable, therefore, that tectonic shift in the habitable zone of a star might be pretty rare.
Dan