When using the Convert To Comments mod tool on a post with child comments, of which the child comments have unattached users (and in this case the post had an attached owner) the unattached children break the conversion.
Example post structure (poor example but simple to follow, post ids are contrived for the purposes of an example)
Q: PostId 5001 - attached owner A: PostId 5004 - attached owner C: - unattached owner C: - unattached owner C: - unattached owner C: - attached owner (author of 5004) Convert To Comment Attach to Post?  [X] convert child comments?
When I do this, it only copies 5004, and none of the C, onto 5001. I would expect it to copy the C and just never attach them to an owner, if it can't track the owners for in the future.