I find it bizarre that 2 Apple stores I visited this weekend had boxed versions of Parallels 10 for sale, yet I can't buy it from the Parallels online store yet. Here's hoping I can tomorrow.
Your link shows the upgrade price. It is not the link to the full non-upgrade program.
Just use that link. Pay 49.99 instead of 79.99.
Its on the "honor" system- there is no check to make sure you have parallels 8 or 9
After you pay, it gives you the key, and the key just works.
That is false, in Parallels 10, it asks for your Parallels 8 or 9 key to activate it with an upgrade key.
That is false
Source: I have done it
My understanding is that it only asks for the older key if an older version is not already installed. (Otherwise it just gets the key directly from the older version.)