Depends on how you go about it.
I use a ball end 2mm hex and everything stays in place.
Some dremmel a section out of the belt guide so you can go straight into the pinion screw without needing a ball end hex driver.
Some drill a hole in the bottom of the chassis so you can get a hex driver up to the pinion screw.

Some take the motor out regardless of pinion size if they haven't figured out the above methods.

A 15t pinion may not even fit through the motor mount plate and you may not be able to put the pinion on the motor and then mount the motor on the plate. You may need to leave the motor in and tighten the pinion.