"[E]mployers may want to transition their pre-Medicare retirees to marketplace coverage -- and provide them with financial assistance to help pay for the coverage or to offset some of the cost-sharing burden. Not only may this approach reduce ongoing costs, but it may also help mitigate or eliminate the Cadillac tax on retiree coverage. Employers that want to provide financial assistance through health reimbursement arrangements will need to proceed carefully to avoid potentially unwelcome consequences."