An upgrade is a new major release (2006, 2007, etc.), not a service pack. You can upgrade all clients or upgrade only a subset so that some clients remain at the old version and others include both versions.
The administrative image
must be updated before clients can update.
To upgrade clients:
Make sure all clients are at the same service pack of the previous release (clients that do not already have the application installed are not upgraded).
Create a new administrative image (do not upgrade or overwrite the old image). To determine the current version of the administrative image, double-click showversion.bat located in the administrative image folder.
SolidWorks Setup Wizard. Running from a client (to capture options) is recommended.
Upgrade all clients to the new version.
Command Prompt. Use the /i and /qb options of the msiexec.exe command. Specify feature properties and global properties that can be passed with the command prompt. For example, to upgrade an existing version of SolidWorks, enter the command:
msiexec /i "<Msi_path>" INSTALLDIR="C:\Program Files\<your_folder>" REGCODE=xxxxxxxx UPGRADESWINSTALL=1 SWMIGRATE="SolidWorks 2006 SP0" /qb
You can delete the administrative images for versions no longer required.