Is it bad to put on new rotors while still using the existing brake pads? Thanks.

 

i wouldnt think so unless your pads are messed up in some way

 

Why are you changing the rotor? If it is groove then you pads are grooved also. If is just warped, wait till your pads are need to be replaced before doing anything. It is generaly not a good idea to not change pads when replaceing rotors. the pads will be worn to the old rotors and you could end up with poor braking, faster wear or noise.