Bicycle rear derailleur pulley size?

Question

What will happen if 2 different pulleys are used? For example, a 13-tooth and a 15-tooth?

Answer 1

Different diameters might not fit. Get the right size for your derailleur.

Answer 2

Well, Shimano does do this on a couple of their cheaper derailleurs. This is to take up more chain so they can provide a wider range of gears.

Retrofitting one size for another is not advisable since it makes no performance difference and could cause clearance problems with the chain and pulley cage.