There could be technical issues there, and with moving comments in general, but I think the major problem is semantic. The comments from the open thread post will relate to the open thread post; even if the post is well-received, it should certainly be rewritten before becoming a top-level post, and then the comments will not necessarily be relevant.

It seems like a much better idea to just take the existing comments into account when writing the top-level post, link to the open thread post for reference, and move on.