change to ufsid format. Should be scriptable.

gistfile1.txt

: cat /etc/fstab
# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/ad0s1a             /               ufs     rw              1       1
/dev/ad0s1b             none            swap    sw              0       0

: dumpfs /dev/ad0s1a | head -n 2 | tail -n 1 | cut -f2 -d'[' | cut -f1 -d ']' | sed -e 's/[[:blank:]]//g'
51928c99a471c440

: swapoff -a
: glabel label swap /dev/ad0s1b

: sed -i'' -e 's/ad0s1a/ufsid\/51928c99a471c440/g' /etc/fstab
: sed -i'' -e 's/ad0s1b/label\/swap/g' /etc/fstab

: cat /etc/fstab
# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/ufsid/51928c99a471c440             /               ufs     rw              1       1
/dev/label/swap         none            swap    sw              0       0