ok this maybe where you went wrong.
when you get to this page first
you need to put in where the new forum is going not the old forum URL.
and you leave everything blank here and put a tick in the box.
then click on coverting an existing board on the next step. which it then usually finds the old board by default, but do check the URL's there.
then the next step be carefull to put the right URL in. it must be where your new board is going.
and this part is up to you.
I know this is rehashing things. but there might be something you missed here.