Completely different? No, they are only slightly different but with a better engine/netcode (I think is the proper term.)
They're pretty significantly different. Whatever the case, we do know keeps are going to work differently and prettymuch everyone has some kind of beef with the old siege system it seems like. Too easy to lose your stuff overnight. Too hard to take if there are defenders with firearrows. Given nobody was happy I think it's pretty worthwhile to discuss what kind of systems we would like to see without the assumption it will be a straight cut and paste.