You can't put your own money into an HRA. You can't take HRA money with you when you leave the company. You do not control when your HRA is used to pay for services. It rolls over, sure, but those three things don't change.
An HSA is a small amount from the company, plus whatever you choose to contribute. It earns interest, which belongs to you. It rolls over. You choose when to spend it and you keep the money if you leave the company.
All of that was in the OE brochure. Looking at that, I don't understand how a reasonable person could assume that any HRA balance would follow them into an HSA. I also don't understand why, if you weren't using your HRA money, why you wouldn't have been in an HSA to begin with. And now you want to go back to an HRA and keep rolling over money that doesn't belong to you?
I'd let it go. I'm no lawyer, but I'm not seeing you being cheated. If you elected an HSA this year with a 5500.00 balance (plus your 500 from the company) you'd have your 6 grand back by the end of the year for about 100 pre-tax dollars a week, earning interest in the name of Somad.