Moving Networks

We are moving our head office from its present location to a new one, the problem is the head office houses all our servers and is connected to 3 other sites around the country.
What I would like to know is the best way to relocate all our servers into the new premises with as little disruption to our 3 other sites that are connected via ISDN and Kilo-Stream link to the head office.
The Servers that we have in the head office are as follows
1 primary domain server
1 Backup domain server
2 Exchange servers
Many Thanks