Upgrading ghost client fails

Hi,

 

My ghost server is version 11 but the client installed on the desktops is much older (i think 7).  I'm trying to upgrade to the latest client using the "Remote Client Install" option in the tools menu of the console, but everytime I do it fails with the message "Ghost installed: There already is version of Ghost installed on this computer".  If i uninstall the client manually the install works fine but that will become time consuming.  Am I missing something?  What's the quickest and easiest way I can upgrade the ghost client on around 400 desktops?

 

Cheers.