The 20khz is the chopping frequency of the chip and has nothing to do with the step or direction signals so nothing has to be changed for it.
If you tie dir- to ground and dir+ to direction pin then you have dir - ground, dir+ +5 for one direction and dir- ground and dir+ ground also for the other direction. That would be the normal way these drives operate.
If you tie dir- to ground and dir+ to direction pin then you have dir - ground, dir+ +5 for one direction and dir- ground and dir+ ground also for the other direction. That would be the normal way these drives operate.