Me. Rev. Stat. tit. 5, § 18534

Any person entitled to receive a disability retirement benefit under article 3, as in effect immediately before October 1, 1989, or under section 1122 of the former retirement system law, as in effect immediately before July 1, 1977, may elect to be governed by this article instead of article 3 by making written application to the chief executive officer within 6 months after adoption of this article by the participating local district. If the election is made, this article shall apply from the date of the person's original eligibility for disability retirement, but any increase in benefits may only be granted from the date of election.