Mailserver migrations, always fun I don't think I have ever completed a large scale migration without some users being affected. I would recommend you consider the following,
Request that the DC route the /32 IP you'll be using in the new rack to the old rack for now so you can alias that IP on the server. Once it's been aliased, you can safely change your DNS to point to the new IP and the old IP will still be there for users who take longer to update their cache. The only downtime you will suffer in this scenario is the time it takes to physically move the machine.
The alternative is to have that single /32 routed to the new cabinet along with the server.
If neither one of these is possible, keep the router in the old cabinet and port forward the traffic to the new mailserver IP until the DNS has updated.
Any reasonable DC will be able to accomodate one of these scenarios. If the physical distance between the two racks is beyond the distance of CAT5, consider creating a VPN tunnel between the firewalls in the two racks to expand your VLAN.
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