I have a 21 node cluster on ec2 (cass 1.2.5). One node was experiencing intermittent network issues. I rsync'ed the data to a new node and replaced the old one. The new machine took over fine in the ring for the old node.
In OpsCenter, it shows the old IP as back "UP" along with "all 21 agents" connected. I have not installed OpsCenter agent on the new machine. Obviously the stats are not being updated in OpsCenter for the replaced node. It still shows the load and compaction stats for the old machine.