Convert Custom Posts to bbPress Topics

We have a small number of custom post-type posts with comments that we would like to convert to bbPress Topics with replies.

I’ve tried a few conversion plugins:
http://wordpress.org/plugins/post-type-switcher/
http://wordpress.org/plugins/convert-post-types/

Both of these convert the post type correctly, but the comments disappear rather than getting pulled along as bbPress replies.

Any ideas?