Just go to your install folder and click on Unregister.exe
After that install all as suggested in the upgrade path pdf file.
You don't need any serial key to install the upgrade.
If nothing helps send a Personal Message to Mad_Fred with the serial keys which you do have, including a proof of purchase.
I'm sure Mad_Fred will help yu out.
Never place your serial key on the open Forum. Really, N.E.V.E.R.