You can add a game name redirector to your project in your project’s Config/DefaultEngine.ini file, under the Engine section like this:
[/Script/Engine.Engine]
+ActiveGameNameRedirects=(OldGameName="/Script/OldGame",NewGameName="/Script/NewGame")
+ActiveGameNameRedirects=(OldGameName="OldGame",NewGameName="NewGame")
Which should allow you to access the class and load your stuff! We’re also working on ways to salvage content where the parent class has gone away. Right now, we block it because of the potential for data loss when we try to load it up, but there are things we can do to let you pick a new base class and recover some of that data. Stay tuned!