So the other day I tried upgrading my main work machine from Mavericks to Yosemite. Since we’re now at 10.10.1 I thought it might work. Not really. It took hours to install, hanging on the “4 minutes left” mark, but got through, finally. Then Mail wouldn’t work, freezing at the “updating mail database” (or something to that effect). Tried it multiple times.
This time I’d been smarter, having done the upgrade on a SuperDuper “sandbox” drive, so all I needed to do to revert was to reboot from the internal drive. Lucky for me that the “update” of the mail database hadn’t destroyed it. Worked fine on 10.9.5.
So I guess I’ll wait for 10.10.2 and try again then. Not that I think it will work.