It's possible to upgrade an ext4 filesystem to btrfs, and it's done the way you're describing. The end result is a btrfs filesystem containing the same file hie

It's possible to upgrade an ext4 filesystem to btrfs, and it's done the way you're describing. The end result is a btrfs filesystem containing the same file hierarchy, with a bonus: an additional raw file whose content is a disk image of the ext4 filesystem (pointing to the same blocks as the files), which allows a rollback.