In VirtualBox, the upgrade from LMDE1 to Betsy worked without any problems. I followed the tutorial Clem's announcement of the upgrade path points to :
http://community.linuxmint.com/tutorial/view/2013
and though it took a while, it went fine. I found that the first apt dist-upgrade didn't complete the upgrade, running the command (as suggested in the tutorial) again did the trick. Once I restarted the system, update manager offered me all the new stuff, kernel 3.16, lots of updated software, cinnamon 2.6 (I believe), etc. On the few occasions I was asked whether or not I wanted to overwrite certain files, I accepted the default (overwrite), no problems there.
The only weird thing I encountered, and it's an extremely minor thing, was that after the final restart, my conky had lost its transparent background. However, a reboot somehow sorted that out. So far, everything seems to be working fine. As I say, that's in a VBox guest. I don't see why it should not go just as smoothly on the host machine - but I'll wait for a day or two and see what others experience before I upgrade for real
