How to turn a Btrfs partition into RAID1:

Create RAID1

mount /dev/sdb1 /mnt
btrfs device add /dev/sdc1 /mnt
btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt

Remove RAID1

mount /dev/sdb1 /mnt
btrfs balance start -dconvert=single -mconvert=single /mnt
btrfs device remove /dev/sdc1 /mnt