Section 90E. If the classification under which a former employee retired has been abolished since his retirement, a city, town or district or the Massachusetts Port Authority, which accepts the provisions of sections ninety A, ninety C or ninety D, may increase the retirement allowance of such employee to an amount as nearly equal as possible to the increases which have been made and which shall be made in the retirement allowance of employees who were retired in the same grade as such former employee regardless of classification.