(a) If on the effective date of participation of the employing department, a person had with a municipality an employment contract that is violated by the membership requirement of Section 852.101(a)(1), the person is not required, but may elect, to become a member of the retirement system.
(b) If a person who is qualified to make an election under this section has been notified that the municipality has elected to participate in the retirement system, or if the person makes contributions to the retirement system, the person is considered to have elected membership in the retirement system unless before the date the municipality's participation becomes effective the person files with the board of trustees written notice of an election not to become a member.
(c) A person who elects under this section not to become a member may never become a member of the retirement system.