dns

we have an existing dns server (primary & secondary). we will be changing our isp next week. my question is how do i go about reconfiguring my dns? considering that we will be using the new leased ip addresses that our new isp provided. also the microsoft exchange 5.5 is installed on the primary dns server. and the microsoft proxy 2.0 server is installed on the secondary server. we have a firewall (checkpoint software). the primary/secondary is resides on the DMZ zone. thanks.