edit client.cpp and where you find 
	Code:
	Message(15, "You have gained a level!  Welcome to level %i!", check_level-1);
 add
	Code:
	if(check_level-1 >= 20) {
  this->SetPVP(true);
}
 right under it and recompile.  the only issue i can see is if they drop below 20, and get it again, they'll get another "You follow the ways of discord." message. 
I just tested this.. works exactly as stated.
farce