[CentOS] Replacing Multiple Servers with One

We are replacing four servers, running mail, web, ftp, and dns, respectively, with a single server to run all four services.

The new server will have a new IP address.

It seems fairly straightforward to redirect mail, web, and ftp services to the new server via DNS CNAMES, but I'm not quite sure about how to do the change for the DNS service itself.

Is there a need to maintain the old DNS server's IP address during a transition, or longer? Via a virtual IP with the old DNS server's IP address on the new machine, perhaps? Or a second NIC with the old address? Or just have the router redirect incoming DNS requests?

Thanks.