A project parameter need to be added to make sure a unique change can be selected

Have you tried using the format project~changeNum? That's the way you should identify changes in this case, because the change number is not unique when importing changes with conflicting change numbers.

Actually, by passing the project~changeNum it would have worked, however the parameter parser after having found the right change to set the topic, then throws away the project information and then just calls the ChangesCollection.parse(Change.Id id) and fails because it uses the index which may not find the imported change by number.

Found the culprit in ChangeArgumentParser: even if the change-id format contained also the project name as project~changeNum, the change resource was created without the project name. By adding it as changeResource = changesCollection.parse(changeNotes.getChange().getProject(), cId, null); the problem is fixed.