Wed Sep 14 16:00:00 EDT 2016

change root fs to btrfs

Mount disk somehow (rescue disk, USB adapter).

fsck -f /dev/sdx1
btrfs-convert /dev/sdx1   # DO NOT DO THIS (1)

mount /dev/sdx1 /mnt ; cd mnt
for fs in proc sys dev dev/pts; do mount --bind /$fs $fs; done
chroot .

change /etc/fstab to point to new uuid
grub-install /dev/sdx

for fs in dev/pts dev sys proc; do umount $fs; done

(1) EDIT:

not reliable.  tried on two laptops, both gave errors that lead to
remount-ro.  use backup & mkfs.btrfs & restore.