As far as I know (service advisor for european brand), it should not invalidate your CPO warranty, but I would make sure that if you do it yourself, at least purchase the parts from the dealership and save the reciept. If there is ever any question on down the line about maintaining the vehicle per manufacturer's specs, then you have that to go on. If you have it done somewhere else, then save the paperwork showing what they did, and at what mileage and date.
Check your paperwork you received when you purchased the car and see if the CPO dictates anywhere that maintenance "must" be performed at the dealership.
If you just don't do it, then if there is ever an issue, they may be able to cite "failure to maintain vehicle to manufacturers specifications" as a reason to deny a warranty claim.