Section 15. All officers and employees of an abolished county or of Hampshire county or of the Suffolk and Berkshire counties’ registries of deeds transferred to the service of the commonwealth or a regional retirement system shall be transferred without impairment of seniority, retirement or other statutory rights of employees, without reduction in compensation or salary grade and without change in union representation, except as otherwise provided in this chapter. Any collective bargaining agreement in effect for such transferred employees immediately before the transfer date shall continue as if the employees had not been so transferred, until the expiration date of such collective bargaining agreement. Nothing in this section shall be construed to confer upon any employee any right not held immediately prior to the date of said transfer, or to prohibit any reduction of salary or grade, transfer, reassignment, suspension, discharge, layoff or abolition of position not prohibited prior to such date.