Hi Ratnavel.
Actually you shouldn't start such a migration lightly. We already have customers doing some migration from jBPM 3.x to other Process Engines. But since basically everything has changed (database layout, API, process definition, with jBPM 5 even some core concepts) it really some change.
Currently we made best experience with a step by step approach (see e.g. http://www.bpm-guide.de/2010/12/24/using-bpmn-2-0-models-with-jbpm-3-a-migration-path-and-more/): Extract the process definition to BPMN and maybe keep the jBPM 3 by a transformation (see e.g. http://fox.camunda.com/doc/#jbpm3Bpmn). Abstract the Process Engine (with a layer like http://fox.camunda.com/doc/#peal). Afterwords exchanging the engine is more a architecture/framework thing and you can migrate process by process. There I think migrating instance data should be avoided if somehow possible, it is really complex :-)
Cheers
Bernd