Question : Class C to Class B/ Domain Controller Upgrade Windows to 2008 R2

I currently have a Class C network with only 5 available IP addresses left.  I would like to move our single Windows 2003 Domain Controller and upgrade to a new 2008 Domain Controller on a Class B network.  Since our current network must be available 24/7, I'm looking for the easiest process to accomplish both tasks.

Answer : Class C to Class B/ Domain Controller Upgrade Windows to 2008 R2

This is really two independant questions...re-numbering your network, and installing a new domain controller.

1) Network re-numbering: I'm assuming you are using a private IP range (e.g. 192.168.1.0 / 255.255.255.0) and that your clients are configured via DHCP, with some static clients. Re-configure your static clients to use the same IP information, but with a 255.255.0.0 netmask.  Make the same netmask change to your DHCP server.  After all clients are using the new netmask of 255.255.0.0 you can begin re-numbering devices to anything between 192.168.0.1 and 192.168.255.254. I don't recommend re-numbering your domain controller (aside from the netmask change, of course).

2) Domain Contoller: Buy a new server, install Server 2008 and promote it to a domain controller. After your Active Directory replicates you can cleanly demote your old server and then remove it - or leave it and have yourself two DC's.