The car has to go on the diagnostic machine for the rear pads to be changed as the electric handbrake has to be disengaged to wind the calipers back.
500x brake pads have loads of meat on them by the way, they last for ages. I was going to change mine when I put my drilled discs on at 18000 miles but there was still around 4mm on the pads.
That’s why the genuine fiat pads are expensive, because they last.