Opened up a 7.5 project, and was greeted with the message "There is a backup project newer than..." . So, I let it open the newer backup, and rename the other to xxx-01. And then got a message along the lines "This project cannot be opened because it was created with Cubase 1.x".
Paraphrasing the messages, but you get the idea. When I opened up the renamed project, I got the same error message.