Network Running low on IP Addresses

Hi

I need some advice regarding IP ranges for one of our networks.
We have a network which is getting bigger and bigger and we are running seriously low on IP addresses.

The main LAN is on IP range 10.0.0.x with a subnet of 255.255.255.0
We have a IP Phone Vlan which we run on 10.0.1.x with a subnet of 255.255.255.0
Backup network runs on 10.0.3.x with a subnet of 255.255.255.0
Other networks on 10.0.4.x and 10.0.5.x all with subnet of 255.255.255.0

The issue I have is the main LAN on IP range 10.0.0.x I am nearly out of IP addresses and need to expand this.  I would like to avoid having to change anything on vlan2 which is running a Mitel phone system as this was quite complex to setup when it first went in from Mitel.

Ideally I would like to put all the PCs onto a separate range as these would be the easiest to move without having to reconfigure any servers, DNS, AD etc.

I have never had to do this before could someone advice on the best way of doing this or if there is a better way to increase the scope.

thanks