There is a chance he did not run new wires if he was able to ascertain that the wire gauge was sufficient to carry 20 amps over the length of the cable. This is a totally valid upgrade though it does obviously require you to be pretty sure you know the length of the entire circuit. If it was Southwire Romex, you can usually tell just by looking at the color of the sheathing on the cable (usually visible in the wallboxes.)