If your bandwith allows it, just set up a proxy server on the old IP. Apache will do this for you just fine. This is also what I do for virtual openacs hosting (The AOLServers are listening on 127.0.0.1, obviously). Works very well.
Add to your apache config (server or virual host section):
<virtualhost *>
servername virtualhost.com
proxypass / http://127.0.0.1:8000/
proxypassreverse / http://127.0.0.1:8000/
</virtualhost>
and in your nsd.tcl nssock section:
ns_param location "http://virtualhost.com"
Otherwise redirects won't work.