freebsd stable - Mar 2012 - upgrade to 8.3 from 7.2 static root mountpoint disappeared

I just did an upgrade from an old 7.2 installation to 8.3-RC2 last
night, and the kernel was only able to mount root from a ufsid label
and could not detect any bsdlabels. The 7.2 installation used static
bsdlabeled disks (/dev/ad0s1a etc). This seemed to be an unexpected
change (I had to get the remote datacenter to connect an ipkvm to fix
the problem), particularly because the 7.2 kernel never loaded
geom_label but somehow the  partitions ended up with labels quite
mysteriously; the box was originally installed from 6.x-R from 2006.