Søren,
Here is what the store says when you get to the PM 5 product page:
"If you already own a license, please login to check your eligibility for an upgrade. [Login here]
Please call Camera Bits Sales ( +1 503.547.2800 ) if you believe you are entitled to purchase an upgrade and do not see that purchase option, if you purchase a full licence, we are not able to refund the difference."
I will add that if you did login and you still don't get the offer of an upgrade, please contact sales@camerabits.com with your purchase details and they should be able to help you out.
-Kirk