The general consensus is that the non-single-weld SS was impractical (Mk1 blew apart!). Then the bands for Mk2 were irregular and couldn't be welded together, according to the video above, so there goes Mk2. Don't know of the timing of the decisions. I've also read reports of significant changes in the fins/flaps, etc. You can read all sorts of info and rumors in the nasaspaceflight.com forums.
I expect most of the first versions will blow up in some fashion because SpaceX is going too far, too fast.