You should copy those comments into the new item, appended to the end of the document (if possible), with the full date/time stamp of the original (old) comment with the username as well. If not possible (for example with audio/video content items), you will need to recreate the comments after the item is created. Then either past in the comments with the full date/user stamp into the comment (easiest but a little messy), or if you are able, after you post the comment, edit the comment to be the original date/time stamp and username as the old posting (if possible).