Section 18. Notwithstanding any other provision of this chapter, all retirees, their spouses and dependents insured or eligible to be insured under sections five, ten B and twelve, if enrolled in medicare part A at no cost to the retiree, spouse or dependents, or eligible for coverage thereunder at no cost to the retiree, spouse or dependents, shall be required to transfer to a medicare health benefits supplement plan offered by the commission under section ten C or section fourteen; provided, that benefits under said plan and medicare part A and part B together shall be of comparable actuarial value to those under the retiree's existing coverage. Each retiree shall provide the commission, in such form as the commission shall prescribe, such information as is necessary to transfer to a medicare health benefits supplement plan. If a retiree does not submit the information required, he shall no longer be eligible for his existing health coverage. The commonwealth shall pay any medicare part B premium penalty assessed by the federal government on said retirees, spouses and dependents as a result of enrollment in medicare part B at the time of transfer into the medicare health benefits supplement plan.