The technically correct answer: You should either have your rotors resurfaced (if they still are thick enough, which they should be at only 20k miles), or replace them with new ones.

Practical answer: If they do not have much wear on them, and they are not warped (no pulsing of the brake pedal when brakes are applied), then you could just scuff the existing rotors with 80-120 grit sandpaper to knock the glaze off of them, and replace just the brake pads. The new brake pads need to wear in with the rotors, and they can't do that on a smooth glazed surface. I did all 4 corners of my 2012 like this (also with Akebono ceramic pads), and they work fine afterwards.

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03 Z4 2.5i
12 X5 35i