Your best bet is to leave the user story under the RBL project, and then create sub tasks that you will later move to the relevant iteration and create a shortcut under the original user story. That way your RBL will reflect your exact progress and if tasks need to be carried over into another iteration, you can create a new task under the user story and close out the task in the previous iteration.
There is a custom action that I can send you that will actually move a work item to an iteration and create a shortcut under the original parent, let me know if you need it.
Another option is to create custom fields on task levels that will indicate that a task is being continued in the next iteration. But this may not be the way you want to work.
Let me know if you have any other questions.