If keeping the project history is important to you, then the only way is to use Paratext 8 to migrate the project. Restoring from a backup file will give you the text and notes and Biblical terms but will not show any history of how the books were edited over time.
Importing books from a PT7 project into PT9 will also not show any history. I also think you cannot import project notes and Biblical terms into PT9 from PT7.