Maybe I did not make myself clear, I would propose to do the migration step by step. The first step indeed doesn't change the engine but prepares everything to change it in the next step. But if all your tests still run after the first step, you know you didn't break anything and have BPMN 2.0 models and a abstraction layer ready to make migrations easier. We just made very good experiences with this in big environments. Didn't say it is th only way.
And for the advertisment: Hy, it is open souce too and I was long time jBPM 3 commiter, so don't blame me for liking this engine ;-)
Cheers
Bernd