Pointless changing the rotors until you have worn the old ones out. After 20k km I doubt you have fritzed them yet. Think they can be worn from 30mm new down to 28mm before replacing.

Mine had 45K km ( With some fun ) on them before replacing and were still 28.5 mm. Jus cos you need new pads doesn't mean the rotors are stuffed.

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Stoner & Webber 2011 World Champions