Try adding bunch of redirectors to your DefaultEngine.ini, that will translate between classes upon loading, and then open unchanged content.
[Engine.Engine]
+ActiveGameNameRedirects=(OldGameName=“/Script/ShooterGame”,NewGameName=“/Script/UT40kGame”)
+ActiveGameNameRedirects=(OldGameName=“ShooterGame”,NewGameName=“UT40kGame”)
+ActiveClassRedirects=(OldClassName=“ShooterCharacter”,NewClassName=“/Script/UT40kGame.UT40kCharacter”)
+ActiveClassRedirects=(OldClassName=“ShooterWeapon”,NewClassName=“/Script/UT40kGame.UT40kWeapon”)
etc…
(not sure which version of game name redirector is correct, one of those will work)