If you redirect requests received at the old site to the new site's IP address you'll have folks hitting your new site much sooner than if you wait for DNS to handle it for you. I have seen examples where clients continue to hit the old IP even after several TTLs have passed. Not everyone plays nicely by the rules...
Redirect should be simple at the topmost level (the maintenance page). A little more difficult would be to rewrite and then redirect deeper requests. I haven't used aolserver's rewrite capability so I can't say what it would look like.
Disclaimer: I haven't actually tried this with aolserver and openacs, but it should work...
