Just back from vacation and saw this go by on the mailinglists of the jBPM project. A possible solution that many of us have encountered for migration of existing process definitions (running processes) over to a new version of the process definition. Check out the overview at Caleb Powell's blog.
It appears that this might be getting pushed into the jBPM project as an apart tool for the 3.x versions, so drop over to the blog and post your support for them to contribute this tooling. Really great stuff!
===================== UPDATE ======================
This looks like it will happen, see the jBPM forum thread.