If you’re upgrading MOSS to SharePoint Server, you have to enter the required license key. Your SharePoint 2010 license key must match your SharePoint 2007 key.
For ex: if your SharePoint 2007 farm is running a trial key, you will need to enter a trial key for SharePoint 2010.
If your licenses don’t match, you’ll get an error like in below example:
After you enter a license key that satisfies the installer, it starts installing the SharePoint 2010 bits.
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Hope this help
Ashi
thanks for the useful information.