I was wondering if you can change rotors between differant brushless motors... will it affect anything if its the same size? also would it increase or decrease power?

A "stronger" rotor (one that provides more magnetic flux) will lower Kv, increase Kt, increase iron losses at a given speed, and decrease copper losses at a given torque.

I assume that you are asking if you can exchange rotors among different Brands? Generally speaking each motor manufacturer's rotors are designed to fit only that mfg's motors.

Yes and no... yes they are the same mfg. there just a 8.5T and a 17.5T. Can i do a rotor swap? The 17.5 is 1900kv while the 8.5 is a 4000kv all i want to do is switch rotors so the 17.5 will have the 8.5 rotor, and if it does fit will it increase my speed?

If they are same brand/model the rotors are most likely identical, so swapping them would have no effect. What brand are they?

Hobbywing