To clarify further:
Parallels.com is no longer directly selling Parallels Desktop for Mac 8 as an upgrade online.
Yes, buyers might be able to find a copy of older Parallels Desktop for Mac products for sale elsewhere.
For users of Parallels Desktop for Mac versions 6 and earlier, there is no upgrade to Parallels 9 available directly from Parallels.com.
Also note that purchase of a Parallels upgrade requires BOTH the Activation Key for both Parallels 9 and the Activation Key for the prior Parallels version.
Assuming you've registered your prior purchases at Parallels.com, they do store the Activation Key for all prior purchased versions with your registered email and password, so be sure to use the same email address for all registrations.
Purchasing the Parallels 9 product or upgrade allows a one-way move or copy of an existing Parallels 7 or Parallels 8 virtual machine into Parallels 9. I successfully moved both a Windows XP and a Windows 7 virtual machine from Parallels 7 to Parallels 9.
Once opened with Parallels 9, virtual machines are no longer readable by Parallels 7.
And there are other virtual-machine solutions not yet mentioned, including VMWare Fusion and VirtualBox. I've used Parallels Desktop for a few basic Windows applications since 2007 (Desktop version 3) and have had no reason to try the other solutions.