A brake rotor replacement is needed if the brake pads wear grooves into the brake rotors (discs). Brake pads, located at each wheel, apply pressure to the brake rotors/discs to slow or stop the vehicle. As the brakes are applied, brake pads come into contact with the brake rotor to slow the vehicle down. The harder the brakes are applied, the quicker the vehicle stops. Since brake pad material wears over time, they should be periodically inspected for wear. If the brake pad material is worn away completely, it causes metal-to-metal contact between the brake rotor/disc and the brake pad causing the brake rotor.disc to be come damaged resulting in the need for a brake rotor resurfacing or replacement.
In order to perform a brake rotor replacement, the wheel must be removed to gain access to the brake rotor. Then brake caliper must then be removed and, in some cases, the wheel bearings may need to be removed as well. Brake rotors/discs need to replaced as a set in order to maintain brake operating efficiency.
You would be best advised to follow the factory recommended vehicle maintenance schedule for how frequently to inspect the brake pads to minimize the need for a brake rotor replacement. While most manufacturer factory warranties and extended warranties only cover the brake pads and rotors for a short period of times, there are maintenance plans that will cover the cost of brake pad inspections, which will cut down on the frequency of brake rotor replacements.