Well, its not as easy as it seems it is.
To change brake pads, you need at least also to service/clean
1) rotor in case for any uneven area or new rotor;
2) piston(s) that pushes the pads to the rotor to stop your vehicle
Reason:
1) Your rotor and pads will have patterns after working together for a while, if you change 1 but not the other might have other stopping issues, like harder to stop or pulsation.
2) Your piston needs to push your pad to your rotor, and after a while, your piston does not move as move, or not moving at all. That will significantly affect the ability to stop of your vehicle.
My 2 cents.