Subject to the provisions of sections 2510 and 2585 of the Education Law, where a board of education, on or after the effective date of this Subpart, modifies the organizational structure of a school in such a manner that instruction in the core academic subjects is departmentalized in a grade or grades previously taught by professional educators deemed to serve in the middle grades tenure area, each tenured professional educator or probationer serving in such grade or grades at the time of such departmentalization shall retain such status and shall be eligible to teach any core academic subject or special subject for which such professional educator possesses appropriate certification; provided that such tenure shall pertain only to grade levels not higher than those formerly associated with the middle grades tenure area in such school district.