Romney's plan to overhaul Medicare involves offering premium supports to help seniors buy insurance on a government-run exchange. The value of the premium support (or voucher, if you prefer) would be equal to the second-lowest premium of all the plans in the exchange, one of which is traditional Medicare. This sort of formulation would inevitably result in the true end of Medicare.
Romney's website states that all the plans "must offer coverage at least comparable to what Medicare provides today." That leaves plenty of wiggle room for companies to come up with plans that undercut Medicare unfairly, such as by offering gym memberships to attract younger, less-sick seniors, which happened with Medicare Advantage plans. Of course, given that anything resembling true regulation is anathema to the Republican Party, it's also likely that "comparable" will result in truly "less". Telling people exactly what has to be in the plan is definitely right out. Insurance companies could even attempt to game the system by offering two plans with lower premiums sold at a loss. By pulling the cost of the government subsidy further and further down over time, traditional Medicare will be destroyed.