I heard today that if you have Medicare Advantage now, it will not be available in 2013.
I just got a booklet encouraging me to join Medicare Advantage.
Medicare Advantage will continue for 2013 [Obama needed the votes, so Medicare Advantage was extended].
However, after 2013, it may undergo significant changes or cease altogether. It will up to the politicians to keep it or end it sometime in 2013 before enrollment period for 2014.
Note that individual states have different plans. Thus, the Medicare Advantage plans that will be available for 2013 in your state may differ from 2012 and/or differ from other state offerings.