Try installing MS Office 2007. It will automatically remove previous versions.i.e. demo of MS office 2007
So, what u are telling is to uninstall one Demo Version one need to Buy a Full version
anyway u are allmost right but u would have to re-frase the Answer
See if you run the Installation source of Office 2007 Demo once agian on a system which allready has office 2007 installed it is suppose to give u three option,
Add or Remove Features
Re-Install the App
Remove the App
so, select the 3rd one