Every probationary employee of a school district which becomes in its entirety a part of a unified school district and every probationary employee of a school district employed in a school located in a portion of a school district which becomes a part of a unified school district, where the whole of the district does not become a part of a unified school district, shall become a probationary employee of the unified school district. The service of the probationary employee in the annexed district shall be considered as service in the unified school district for the purpose of any law relating to the classification of persons employed in school districts in positions requiring certification qualifications.