In a school district which becomes unified, effective for all purposes, on July 1, 1980, certificated employees who are employed for the school year 1979–80 by a union high school district included in part in the territory within the newly organized unified school district shall be entitled to be employed by the unified district for the school year 1980–81 only if such employees were employed within the schools being acquired by the unified district. However, the unified school district may, but is not required to, employ additional employees of the union high school district to the extent the unified school district agrees to employ such employees.