LUKS/cryptsetup knows how to in-place encrypt and decrypt if you give it a place to put some metadata (16 MB in my experience):

    cryptsetup reencrypt --encrypt -q --header /some/where/file /dev/mydev
    cryptsetup reencrypt --decrypt -q --header /some/where/file /dev/mydev

(Just used that to encrypt a server disk during shipping and decrypt it afterwards once it reached its destination datacenter)

I haven't looked into it but it might be possible to convert a non crypted disk to a cyrpted one.