The password can be changed using the couchbase-cli tool.
shell> ./couchbase-cli cluster-init -c 127.0.0.1:8091 --user=[CURRENT_USERNAME] --password=[CURRENT_PASSWORD] --cluster-init-username=[NEW_USERNAME] --cluster-init-password=[NEW_PASSWORD]
SUCCESS: init 127.0.0.1
More details about the couchbase-cli tool can be found here: http://www.couchbase.com/docs/couchbase-manual-1.8/couchbase-admin-cmdline-couchbase-cli.html