The servers are getting pretty old now so I reckon they will be replaced - but hanging on to the SAN box.
Microsoft support for XP terminates April 2014, so an OS upgrade is required - likely Win 7.
Logically the servers and OS upgrade would be at the same time - the new servers are mostly pre-configured, as soon as one is installed you start rebuilding the workstations; at the same time copy Learning resources etc. from old to new server. The SIMS database will go into the cloud, freeing disk space on the old SIMS Server.
The C2K/Northgate way may be different of course.