I updated my 7.5 Cubase to 8 then to 9. I used two versions of cubes. I updated the mac os system and can not install the 8 version, for me it is more convenient. When activated, says that the key is no longer valid because of the update 9. Please tell me how to cope with the problem. 
Follow the instructions for “reactivation”
Regards 
Dear friend, I go to my steinberg and I do not have old versions there that I could choose to reactivate … there is only version 9 … this is the whole problem, I do not understand where the two old versions are gone …I click on the reactivation, it tries to reactivate the version which I never bought, 5 cubase …
Open a support ticket from your account page.
You should be able to run the old versions if you have a full version 9 license.
That’s what I’ve heard a lot of times, not sure how to activate it though.
Good luck!