I had a msdn registered copy of Expression Studio installed on my machine and recently got a paid licensed copy. I uninstalled it and removed what I could find in the registry that looked related. However every time I try to install Expression to use my new license it doesn't prompt and just installs. It appears that somewhere the old registration code is being stored somewhere however I am unable to find where
You shouldn't worry. MSDN subscriptions deliver Release version Licenses. You have now acquired in effect two licenses for Expression Studio.
Which license number you input is meaningless. As you can see, your paid copy is fully licensed by your MSDN subscription license. As for the actual location of these licenses, they should be in the registry or in special files in your system folders. The actual specifics of which are never described by Microsoft, for the obvious reasons.
magical jelly bean may be able to find the serial number you're using. they have extensive ms product support