If immediately before 1st April 1974 there is an existing joint planning board for a National Park comprised in two or more existing counties and on that date the Park will be wholly comprised in one new county, the Secretary of State shall by order reconstitute that board as a special planning board to discharge the functions to which this Part of this Schedule applies as respects the area of the Park, and any enactment relating to joint boards constituted by an order under F2section 2 of the Town and Country Planning Act 1990 shall apply to a special planning board reconstituted under this Part of this Schedule and to the order reconstituting it as it applies to a joint planning board constituted under that section and to the order constituting it and shall so apply as if the area of the Park were a united district, but with the substitution of references to the council of a new county for any references to the constituent authorities.