How to boot alternate installation?

[james]

I have a system that boots 9.0+patches (claims to be 9.1-PRERELEASE but 
that's a stretch in this case) off an ssd on sata0 - I needed some mfi 
patches.

Now that mfi is working, I have a raid1 pair on the mfi controller, and 
I've unpacked 9.1-RELEASE onto it using bsdinstall.

Now, the crappy PERC5 and the AMI BIOS in my little AMD mobo don't 
really see eye to eye and I've had a lot of trouble coaxing it to boot 
from the RAID volume (the RAID card initialises late it seems).

I'm happy to keep /boot on the SATA SSD for now.

Is there a straightforward way to configure (the menus to) boot from 9.1?

I'm happy enough to rename /boot on the SSD and copy over the contents 
from 9.1 (really the old 9.0+ system is there as insurance for the moment).

Is the simplest mechanism to do that and override rootdev in loader.conf?

It seems that there are a number of variables with "/boot/..." paths so 
its not so easy to switch between (say) /boot90 and /boot91.