luksipc is the LUKS in-place-conversion tool. It allows conversion of plain volumes to LUKS-encrypted volumes.
luksipc was created before any alternative from dm-crypt/cryptsetup/LUKS side was available. This is not the case anymore. Therefore I recommend switching to cryptsetup-reencrypt, which is properly maintained and tested upstream even when the format of the LUKS header changes (to my knowledge, this has at least happened twice and can cause luksipc to catastrophically fail, i.e., destroy all your data in the worst case).
All documentation is available at https://johndoe31415.github.io/luksipc/.