Mitt Romney's plan for Medicare is vague, probably deliberately so. The less you divulge of a plan some people won't like, the less likely you are to lose the votes of those people.
Patrick Caldwell summarizes what we know of it. It pretty much comes down to privatizing the system so that the government-run system loses strength and the funding it would get goes to privately-owned, for-profit insurance plans. And the elderly would have to pay a lot more out of their own pockets to get comparable care and coverage. Okay, so Grandma might not be able to afford all her prescriptions but we do have to help the insurance companies make more money and keep taxes low for the wealthy.