When choosing a difficulty, hold down X for a while and eventually you'll be given the option to play the game in Extreme difficulty. This mode will only appear if the game has been updated to v1.0.3c or later.
Rapidly press the ZR button to switch between Modern and Retro graphics. As the horizontal wipe effect moves back and forth, it'll gradually slow down and eventually freeze in place, leaving the left of the screen in Modern mode and the right of the screen in Retro mode. How much of each side is present is dependent on the timing of the button presses and can be manipulated with practice.