Subject to the limitations in this article governing the creation of the unit previously established, the supervisor, by entry of a new order after a public hearing, may require unit operation in a pool, or a portion thereof, which embraces a unit area established by a previous order. Such new order, in providing for allocation of unit production from the enlarged unit area, shall first treat the unit area previously established as a single tract, and the portion of 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 therefor in the previous order.