1). Over the first 10 years, it would make relatively small changes to Medicare by repealing most of President Obama's health care law. That would re-open the Medicare prescription donut hole, and deep-six new wellness benefit guarantees for seniors.
2). After 10 years, it would begin phasing out traditional Medicare hospital insurance and replace it for newly eligible beneficiaries with a subsidy to buy regulated insurance on an exchange. The value of the subsidy would shrink over time relative to health care costs leaving seniors holding the bag for more and more of their own medical bills. Still, this is a policy Cornyn himself agreed has a lot in common for with the health care law, for people under 65. Nobody claims that the health care law is anything like Medicare.
3). After that 10 year window is closed, those who already have Medicare would be allowed to retain traditional fee-for-service Medicare. Republicans argue that they'd see no disruption. But as explained here, over time that pool of people would age and shrink, leaving the traditional Medicare program without the purchasing power to entice providers to care for Medicare patients. Over time, that plan would erode until it disappeared altogether, leaving us with a fully privatized insurance program for all eligibles.