Upon application by an owner the commission, by order, may, in the same manner and subject to the same conditions as in an original order, provide for the unit operation of a pool or pools, or parts thereof, that embrace a unit area established by a previous order of the commission. Such order, in providing for the allocation of unit production, shall first treat the unit area previously established as a single tract, and the portion of the unit production so allocated thereto shall then be allocated among the separately owned tracts included in such previously established unit area in the same proportions as those specified in the previous order.