Torsten R,
I have talked about this and more at JUDCon last week, see slides here: http://www.schabell.org/2010/10/judcon-2010-berlin-get-your-bpm-ducks.html
Watch this project space (migration tooling), we hope to have initial release by end of this month with focus on jBPM3 -> jBPM5. For jBPM4 -> jBPM5 we would welcome contributions as this is not our initial focus.
Regards, erics