I'm not sure what discount you are talking about, I assume you are talking about buying an upgrade license versus a full license, in that case, I don't see why you can't purchase an upgrade edition of Windows 7 Ultimate. Their own website clearly states:
Do I buy an Upgrade or Full license of Windows 7?
All editions of Windows XP and Windows Vista qualify you to upgrade. So, if you're running either on your PC today, buy a package labeled "Upgrade".
Keep in mind you'll also need to do a custom (clean) install since you are coming from XP to 7.
You mentioned you have the serial number only, you might be interested to know that the upgrade discs allow for a clean install granted you may have to do a double install. Honestly, the upgrade editions have an unusual amount of faith in the consumer, more than you would expect from Microsoft. I don't see any reason why anyone would really have to buy anything other than the upgrade edition, granted you don't have a guilty conscience. The following article explains the process.