I had some luck. I let it finish, but while I could see the new client, I couldn't edit it. Closing and re-starting the client fixed that.
So I started it, it loaded a few new devices and then wanted to add a new base system but couldn't find any drivers for it. I rebooted a couple of times, always with the same result, and sometimes the new hardware wizard just froze.
Eventually I just rebooted, cancelled the wizard and selected to upgrade VMtools. It worked!
And so far the server is fully working.
Hopefully it will keep on going, but I won't lose any mail if I have to go back to my old server.
I also have an old DOS guest imported and working too, though I just noticed a glitch that will need looking into.