Vista - Updating Vista Home Premium to Ultimate doesn't work

Hello guys

I have a laptop with Vista Home Premium installed and bought
Vista Ultimate directly from Microsoft in Dublin - no update but the full
boxed version. How can I update my existing version to Ultimate without
losing any data? My Ultimate DVD complains that a later version of Windows is
already installed and it abort the installation process. I wonder what
happens if I simply boot from the DVD and start the installation using the
existing partition etc. Will I lose my settings or will only different files
be updated? Exchanging the serial number did not work.

Just for clarification: I did not buy the AnyTime Upgrade which I heard
would come with an update tool. I might need that particular tool as well but
can't find it and wouldn't want to try it and lose all my data.

I hope that my situation isn't too unique and someone has experience with.

Thanks a million for any help!
Daniel