FreeBSD binary upgrade / gmirror

[rihad]

I have a FreeBSD 5.3 box and its two HDD's are setup as one GEOM mirror 
unit /dev/mirror/gm0 (c.f. http://people.freebsd.org/~rse/mirror/). Now 
Now I want to do a binary upgrade to FreeBSD 5.4, but the new 
sysinstall's disklabel editor only recognizes the IDE disks and does not 
consider the GEOM mirror (mentioned in /etc/fstab, btw). How do I go 
about this? Thanks.