Nah id just true the old one You're not bending past its elastic limit ( you need to bend it past the point where it'll just rebound and return to its original /bent shape ) Put the rotor on the bike, adjust the brake to a place on the rotor that looks true; then get a small adjustable spanner.... bend it where it hits the caliper/pads Generally you have to bend it as far as it is out of shape, past where it would be true. For example, if it has been bent by 10mm to the right past straight, bend it 20mm to the left.