Trying to give something back to the Community...
I'm trying to upgrade a Secondary Site but the option to upgrade the Site is greyed out. I'm seeing this in the ConfigMgrSetup.log:
Setup has detected an existing installation of the administrator console. This server seems to be a site server for a secondary site.
Because the current installation is a different version, you do not have the option of modifying the installation. [17/10/2012 - The WMUG Link Below No Longer Works - Please click HERE to access the Article]
Read the complete post at http://wmug.co.uk/blogs/cliffs_blog/archive/2010/08/10/faq-sms-sms-2003-upgrade-to-configmgr-2007-quot-setup-has-detected-an-existing-installation-of-the-administrator-console-quot.aspx