I just followed those steps and all worked perfectly.
As for step 18, you just don’t need to change the GlobalDefaultGameMode line, but you still have to change the other lines, like ActiveGameNameRedirects and others.
Note however, that in this case, unlike step 8, you only change the exact names. For example (oldname = FPCPP):
[/Script/Engine.Engine]
+ActiveGameNameRedirects=(OldGameName="TP_FirstPerson",NewGameName="/Script/**OLDNAME**")
+ActiveGameNameRedirects=(OldGameName="/Script/TP_FirstPerson",NewGameName="/Script/**OLDNAME**")
+ActiveClassRedirects=(OldClassName="TP_FirstPersonProjectile",NewClassName="FPCPPProjectile")
+ActiveClassRedirects=(OldClassName="TP_FirstPersonHUD",NewClassName="FPCPPHUD")
+ActiveClassRedirects=(OldClassName="TP_FirstPersonGameMode",NewClassName="FPCPPGameMode")
+ActiveClassRedirects=(OldClassName="TP_FirstPersonCharacter",NewClassName="FPCPPCharacter")
To:
[/Script/Engine.Engine]
+ActiveGameNameRedirects=(OldGameName="TP_FirstPerson",NewGameName="/Script/**NEWNAME**")
+ActiveGameNameRedirects=(OldGameName="/Script/TP_FirstPerson",NewGameName="/Script/**NEWNAME**")
+ActiveClassRedirects=(OldClassName="TP_FirstPersonProjectile",NewClassName="FPCPPProjectile")
+ActiveClassRedirects=(OldClassName="TP_FirstPersonHUD",NewClassName="FPCPPHUD")
+ActiveClassRedirects=(OldClassName="TP_FirstPersonGameMode",NewClassName="FPCPPGameMode")
+ActiveClassRedirects=(OldClassName="TP_FirstPersonCharacter",NewClassName="FPCPPCharacter")
**+ActiveGameNameRedirects=(OldGameName="/Script/OLDNAME", NewGameName="/Script/NEWNAME")**
You must have commited a mistake somewhere.