How can I change Couchbase server IP / DNS?

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)
Rename it
Add it back in.

See for details of this - the steps vary slightly depending on the version of Couchbase you are using.