How to upgrade Windows 8 pro purchased from Microsoft during grace period to 8.1

Hi,

I have purchased Windows 8 directly from Microsoft during the grace period shortly after release where it was sold at a lower price. It turned out to be a Windows 8 pro.

Now the Windows Store doesn't show the upgrade because I'm a 'pro' user, and Microsoft website tells me to turn to my administrator. Which in my case is Microsoft.

I do not have Technet or MSDN subscriptions nor am I a professional user.

Downloading from the page 'install with a key' doesn't work, because the Win8.1 installer does not accept my Win8 key.

How do I get my upgrade ?