I'm still not confident enough to try it on my real system as yet, as I need to check out a few things.
What exactly does mdadm --zero-superblock /dev/sda1 do? Does this totally destroy data on that mirror? I'd prefer to keep it intact but unused at that point in case anything went wrong, but without doing this it seems to treat the A drive as the master and we end up back with the upgraded system we dont want.
It also seems a bit brute force to have to actually drop the md volume, and recreate it. At this point I was hoping there was a 'rebuild using A' option instead.
Has anyone who knows mdadm got any comments or a better/safer way of doing this?