Change Difficulty During Game (Japanese Version)
Note: The game must have been previously completed on the hard difficulty
setting. At the title screen, press R1, L2, R2, L1, X, Triangle, X, Square. If
you entered the code correctly, you will hear the sound of a zombie. Now while
playing a game, hold R1 + R2 + L1 + L2 and press Select to change the difficulty.