title: Changing weld joint pose
body: <p>I've constructed an MBP with two objects, A (welded to world) and B (unwelded). B sits on top of A. After simulating for a while, I'd like to remove A (or otherwise disable contact) to allow B to begin falling.</p>
  <p>It seems I should be able to do this by either (1) changing collision filtering on the fly or by (2) changing the pose of the weld joint. Haven't investigated (1), but (2) seems to be out- there does not appear to be a way to alter the pose of a weld after construction. True? If true, why not?</p>