Downtime of the server is not a problem…
The IP address / hostname of a cluster node is pretty intrinsic to it’s identity in the cluster, so to change it you need to:
Remove it from the cluster (and rebalance)
Add it back in.
See http://docs.couchbase.com/couchbase-manual-2.5/cb-install/#using-hostnames for details of this - the steps vary slightly depending on the version of Couchbase you are using.