An order establishing development units for a pool shall cover all lands determined or believed to be underlaid by such pool, and may be modified by the department from time to time to include additional areas determined to be underlaid by such pool. When the department determines that it is necessary for the prevention of waste, or to avoid the drilling of unnecessary wells, or to protect correlative rights, an order establishing development units in a pool may be modified by the department to increase or decrease the size of development units in the pool or to permit the drilling of additional wells on a reasonably uniform plan in the pool.