You should be able to select all of them in the Gallery window, export them to another drive, then open the new project in the new database, and import them all as new PowerGrades. If you make sure you use the "Use Labels on Still Export" option, the PowerGrades will retain those names.
I just had to do this at the first of the year when I started a new 2019 Project Database, and it was a very simple matter to bring across the old PowerGrades.