How to predict drive number change for 7.3->8.1 upgrade?

[Michael Sperber]

I just upgraded my desktop system from 7.3 to 8.1, and the main hard
drive, which was /dev/ad6 before is now /dev/ad10.  Consequently, the
initial boot failed when trying to mount the root file system from ad6.

The desktop system is now fixed, but I also have a rented server with
only a serial console, and I worry that the upgrade is going to leave me
with a dead machine.  Is there any way to predict how the drive number
changes?  (Why does it change at all?)  If so, what's the proper way to
tell the system the initial root device *before* rebooting?

-- 
Regards,
Mike