In a district which becomes unified, effective for all purposes on July 1, 1980, certificated employees employed for the school year 1980–81, who were employed for the school year 1979–80 by a union high school district included in the territory within the newly organized unified school district, may be placed on the salary schedule of the unified school district and be given credit for all service as an employee of the union high school district for years of experience as an employee of the union high school district. Their date of first paid service as a probationary employee may be the date established by the union high school district.