How to create query using IQueryDescriptorWorkingCopyManager?

I use one exist query and modify the expression, but seems queryDes.setExpression(expression) does not work.
Anyone can help?

ItemQueryIterator<IQueryDescriptorHandle> iterator=
WorkItemQueries.queryDescriptorByName(auditableClient, projectArea, "all work items");
IQueryDescriptorHandle queryhandle= iterator.next(null);
IQueryableAttribute similarityAttribute= findAttribute(projectArea, auditableClient, IWorkItem.SUMMARY_PROPERTY, null);

IQueryDescriptorWorkingCopyManager queryWorkingCopyManager = queryClient.getWorkingCopyManager();
queryClient.getWorkingCopyManager().connect(queryhandle, null); QueryDescriptorWorkingCopy querywc = (QueryDescriptorWorkingCopy) queryWorkingCopyManager.getWorkingCopy(queryhandle);
QueryDescriptor queryDes = (QueryDescriptor) querywc.getQueryDescriptor();
AttributeExpression expression= new AttributeExpression(similarityAttribute, AttributeOperation.EQUALS, "aa");
queryDes.setExpression(expression);