Also see:
http://www.windowsitpro.com/article/ip-addressing/changing-your-network-s-ip-address-scheme.aspxI would:
1 Find all static address
get a list of all devices that have static addresses. Routers, Firewalls, Wireless Access points, server and the like.
2 Plan you Private IP range
Use this opportunity to give your network space to grow in the future. Maybe choose a range like 172.16.16.0/21. This will allow 2048 hosts with valid IP addresses of 172.16.16.1 to 172.16.23.254 (mask 255.255.248.0)
Now decide which static addresses you will use for the devices in the list compiled above.
3 Set your PCs to use DHCP
Make sure all PCS are DHCP enabled and reduce the lease time on the server to 1 day.
4 Change static addresses (do this on a Saturday)
Have a PC with IP address of both old and new range so you can connect to devices and servers whether they have a new address or not.
Change your routers and firewalls (don't forget NAT rules). Change your servers at the same time setting the right gateway and DNS/WINS addresses. Delete all DNS records pointing to old range and purge you WINS databases. Configure your DHCP server with new scope.
Once you have name resolution correct on the servers and all your inbound NAT rules work (like SMTP mail is still working) then the PCs should pick up the new address range from your DHCP server on Monday morning. You might want to test things on Sunday for a few PCs.