Hi all, as far as I know, you also have to edit /etc/crypttab.
If one has forgotten to encrypt a partition, the easiest way is, to boot from a livefile system. Then backup the whole content of this partition to an external partition. Note, this should be a ext3 or ext4 partition, so you preserve all hard- and softlinks. I suggest for using rsync doing this. Then you can encrypt and format this partion and rsync everything back. Doing so, I never lost data or settings. Hope this helps. Best regards Hans