Windows XP and Windows Server 2003 are out; they are no longer supported by SolidWorks. The following compatibility chart is taken from http://www.solidworks.com/sw/support/SystemRequirements.html.

SolidWorks Network License Server

Operating Systems	SolidWorks 2011	SolidWorks 2012	SolidWorks 2013	SolidWorks 2014
Windows 7(SP1 required for SolidWorks 2014)
Windows 8.1 (64-bit only)				(2014 SP1)
Windows 8 (64-bit only)
Windows Vista
Windows XP
Windows Server 2003
Windows Server 2008
Windows Server 2008 R2
Windows Server 2012

Notice that Windows Server 2012 R2 is not yet supported, and if the past is any indication it will not be supported for at least a year.  However as of SolidWorks 2014 SP1 Windows 8.1 is supported.

If you are still running SolidWorks 2013 and would like to move to Windows Server 2012 you can install SolidWorks 2014 SolidNetWork License manager, it supports previous releases of SolidWorks.

There are 4 main steps to moving to a new license server:

1. Install SolidNetWork License Manager on a new server
2. Point clients to new server
3. Deactivate the old license manager
4. Activate the new license manager
5. (Optional) Uninstall the old license manager or decommission the old server

Install SolidNetWork License Manager

The license manager is installed from the same media as the client.  When the Welcome screen appears select the option for Server products, check off Install SolidNetWorks License Manager.  If you are updating an existing server this option will be the default. Click Next.

Wait for SolidWorks to do all of its checks.  If it has difficulty with this you will need to open up your firewall to allow for the communication to occur.  If it is successful then activation should go smoothly, if not then prepare to do the activation when users are not going to be using SolidWorks.

You may be prompted to download the latest version, for the license server it is usually acceptable to stick with the older server packs.  Click Next.

Enter your network license serial code(s) in the textbox.  Choose an alternate installation location if you want. Click Next.

The summary screen gives you another chance to review all the previous settings and make changes if necessary. Click Install Now.

Installation is complete but that is only the end of Step 1.

Point Clients to new Server

Open the SolidNetWork License Manager on each client.  For version 2014 it can be found under Start, All Programs, SolidWorks 2014, SolidWorks Tools, SolidNetWork License Manager 2014.

Click the Server List Tab and click Add.

Type in your port @ server name (the port will be 25734 unless you changed the configuration on the server).  Click OK.

You may see the following dialog.

This indicates that the server service is not running or a firewall is blocking communication.  Click No and determine what the problem is.  You can review this help topic if it is caused by the firewall. http://help.solidworks.com/2013/English/Installation/install_guide/c_using_snl_licenses_w_firewalls.htm

Otherwise you will see the server listed.  Click OK.

Repeat this process for all clients.

Deactivate the Old License Manager and Activate the New License Manager

This older post still answers how to do this

http://www.javelin-tech.com/blog/2012/01/the-last-step-to-update-the-solidnetwork-license-server/

Deactivating and Activating are the same process, except to deactivate instead of selecting Activate/Reactivate a software license you would select Transfer a software license.