ip traffic forwarder

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We are moving services from one building to another and in the process 
will relocate about 20 servers, which involves changes to the IP 
addressing/routing scheme. We're 98% certain that simple changes to DNS 
will take care of migrating services to the new IPs, but historically 
we've had badly written applications or configurations that hardcode IPs 
in.

The plan is to move the servers to the new building under the new IP 
addresses, and then assign an OpenBSD box on the old subnet the old IP 
address of the server that was moved so we can monitor traffic to that 
IP and find hosts that are still using the old address. This should help 
give us a jump on things that are broken from the move.

The question we have is: can we also redirect traffic coming into the 
old IP address to the new address so as to not see interuption of 
service until we fix the problem? What'd we'd envisioned was doing 
something like a binat rule to translate traffic between the old and new 
IP addresses. The difficulty is that the new IP address isn't located 
"behind" the OpenBSD box and it would seem that it may be difficult 
given the path that traffic would take. Is this setup possible? Is there 
another way to accomplish this (SSH port forwarding, a generic TCP 
proxy, etc.) or are we basically unable to do it and should be happy to 
at least monitor traffic to the legacy IPs?

TIA for any suggestions.

DS