The migration tool probably won't be released at the same time as the 5.0 final version, but have a slight delay (of 1 or 2 months). I suggest you check the progress on http://sourceforge.net/apps/trac/jbpmmigration/
The first focus of the migration tool is from jBPM3 to jBPM5. However, we hope that the community can assist in creating something similar for jBPM4.
The changes between jBPM3/4 and jBPM5 can be quite significant, but we believe it will be worth it, and make sure we can keep supporting the new API and process format for many years to come. jBPM5 is based on the BPMN2 standard and the knowledge-based API will allow you to combine this with powerful rules and complex event processing in a similar fashion.
Kris