Office Customization Tool

I am trying to upgrade from 2003 to Office 2k7. I can open the OCT but it won’t remove previous versions of office. I get a “Upgrade patch cannot be installed” error. I have the MSP in the root of the folder where I will run it from. Is it because it’s a patch and not a true install??? I’m confused as it has provisions to remove previous versions of office. HELP!!! I got 150+ users who will need to do this soon! TIA