well, i think with the flush you shouldn't need the transplant, and that would be the point - to avoid having to shock the plant.
the flush should wipe out excess salts and nutrients and make the soil perfect for adding your choice of bloom nutrients. if you put it in new soil, you'd have to flush that quite a bit to get rid of the nutrients that generally come in the soil.
the general rule for flushing is 2-3x volume of container, but it's not critical.