If at any time after the adoption of a redevelopment plan for a project area by the legislative body, it becomes necessary or desirable to amend or modify such plan, the legislative body may by ordinance amend such plan upon the recommendation of the agency. The agency recommendation to amend or modify a redevelopment plan may include a change in the boundaries of the project area to add land to or exclude land from the project area. Except as otherwise provided in Section 33378, the ordinance shall be subject to referendum as prescribed by law for the ordinances of the legislative body.