It is allowed to add new optional InstanceDeclarations using the ModellingRules Optional and OptionalPlaceholder.

Old OPC UA Servers will just not provide those, and new OPC UA Clients will not expect them to be available in all OPC UA Servers, since they are optional. Since OPC UA Servers are allowed to add anything to the instances, an old OPC UA Client can also easily connect to a new OPC UA Server, potentially just ignoring the new information.