Delete from the modded one these tables :

phpbb_posts
phpbb_topics

etc, and maybe the config of the board, It's first table. There is written when did your board started ( it can be seen in ACP, Board started : xx.xx.xxx )

From the old one you would need only posts and topics. And users of course.

So you go into your old database, go export. Take only those tables I told you. And import them in the modded one.