btrfs is marked as experimental, so it's not recommended. But it's your machine... I'd make sure I have backups of everything I cared about.
If it's just system data that you have ( ie from packages) then a fresh install is the way to go. You can keep your existing configuration by backing up the ones you want in
The other way to do this is, if you have an external drive, you can
cp -rfp / to
media/your_external_drive. After you unmount the disk, you can reboot into a live cd with the btrfs utils and
mkbtrfs /dev/sda ( or whatever dev your disk is on ).
This should preserve grub if you have installed it to the MBR. Then you can plug in your external media and copy back the files. Making sure to change /etc/fstab from ext4 to btrfs.
You could also do the same thing by splitting your drive in two as you mentioned. However if you wish to format your second partition as btrfs before copying, and use it as the root file system, you will also need to change your GRUB configuration to look at the new partition.