If you look on this page and scroll down, it still shows the upgrade from the CPTK and either PT9 or 10 as an option to upgrade to HD12 (in the description).
It appears that you select the HD12 upgrade regardless of which qualifying product you have, and I assume an activation code will be sent to you that works for any of the various possibilities, which would include upgrade from HD9, HD10, HD11, HD12, and the CPTK+9 or 10 combo, to the HD12 with year of support and upgrades. The reason HD12 is mentioned as being upgradeable is for those just now buying a new HD hardware system. They would get HD12, but no support plan or added features throughout the year, so they would also need to buy the support plan to get those, even though they already got HD12 with their new system.
https://shop.avid.com/ccrz__ProductD...03&&store=shop