ReferenceTypes typically restrict their usage by limiting which SourceNodes and TargetNodes are allowed. Currently such restrictions are only specified in text in specifications.

Changing the rules, either allowing more or fewer restrictions, should be avoided. However, OPC UA Clients should consider the results when following References (e.g., the ReferenceDescription containing NodeClass and TypeDefinition) and they should be able to handle unexpected results. Therefore, changing the rules is not forbidden, as long as it does not contradict other parts of the information model e.g., because a TypeDefinition is using the ReferenceType in a way that is no longer allowed in the new version.