PSO2 is a fun game, but it doesn't feel like the original game at all to me.
Yes there are similar concepts but the gameplay is so much different that I don't really feel a connection between the 2 games.
What you can do is :
Rename the PsoBB.exe to PsoBB_old.exe
And rename PsoBB.pat to PsoBB.exe then run that one.
Normally it shouldn't be done that way if you run it as admin though.
But it's possible the antivirus doesn't allow PSO to remove the old exe and replace it with the new one.