I think the old URL should definately have either an HTTP redirect or forward page.

The first couple of days the board was back I was picking up the URL from my favorites and assumed it was still down.

It was only when I opened the main page to look for status information that I found the new board URL.

Any site with a referal to the (old) board will fail, and people will assume that the board is no longer operative.

The redirect should also handle external links to old threads.