I know this is something that comes up every now and then as something that's wanted.
Thinking of how someone would start working this out, I came up with this outline. The challenges are:
1) Selecting the topic you're moving the reply to. If you have a ton of topics, there has to be an easy way to drill down through categories and forums and finally select from a list of topics. If you're moderator is doing this, the selection process has to account for the various config settings.
2) Working through all the admin/cleanup stuff. All of this is in post_info.asp, so it's not like you have to work from scratch. The challenge is do you try to work this into post_info.asp (ugh!) or do you build your own frankenstein script using what you need out of post_info.asp?
So here's what I've outlined so far.
If anyone wants to run with it, by all means go for it. I don't really have plans in the immediate future for this. 
Dim oldReplyID
Dim newCatID, newForumID, newTopicID
oldReplyID = Request("replyid")
'Think of some method to have the admin select the
'Topic this reply is being moved under. Popup window?
'The admin could select forum, then browse through the list
'of topics and select the one which populates the main window?
'If you at least have a topicid, you can always query for the rest
newCatID = ?
newForumID = ?
newTopicID = ?
strSQL = "UPDATE " & strActivePrefix & "REPLY SET" &_
" CAT_ID = " & newCatID &_
", FORUM_ID = " & newForumID &_
", TOPIC_ID = " & newTopicID &_
", R_LAST_EDIT = '" & DateToStr(strForumTimeAdjust) & "'" &_
", R_LAST_EDITBY = " & MemberID &_
" WHERE REPLY_ID = " & oldReplyID
my_Conn.Execute(strSql),,adCmdText + adExecuteNoRecords
'Technically we should check to see if this reply was the
'last reply in the old topic record and adjust
'T_LAST_POST_REPLY_ID & T_LAST_POST_AUTHOR if needed.
'Same for the new topic record, see if we need to update it.
'For simplicity's sake... (skip?).
'Do we check the old/new forum (if different) to see if this
'affects the F_LAST_POST_REPLY_ID?
'Then there's the topic moderation and auto-approving if needed.
'Do we email the author?
'Dont forget about updating T_REPLIES in the old & new topic records.