public NodeList changingVersion() {
        return matching(node -> node.status().vespaVersion().isPresent() &&
                                node.allocation().isPresent() &&
                                !node.status().vespaVersion().get().equals(node.allocation().get().membership().cluster().vespaVersion()));
    }
Returns the subset of nodes that are currently changing their Vespa version
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/NodeList.java