The board may:
(1) adopt rules relating to the seniority of district employees, including rules for a retirement plan based on seniority; and
(2) give effect to previous years of service for those employees continuously employed in the operation or management of hospital facilities:
(A) constructed by the district; or
(B) acquired by the district, including facilities acquired when the district was created because of former Section 2, Chapter 658, Acts of the 64th Legislature, Regular Session, 1975.