Hi Anath.
Q quick anser, but it seems I am not really welcome in this thread (even if I don't get why, I just told what we did in the projects at customers and pointed out to some sources of open source components. Where is the big advertisment again??).
Abstracting the API is possible but not that easy. We did a small POC with the open source abstraction layer we wrote (http://fox.camunda.com/doc/#peal, http://fox.camunda.com/doc/fox-peal/). But you have to write an own binding, which is a bit of work. Biggest problem I think is operations, since they have to care about instances in different engines, which make the already not easy situation much more complex.
Instance migration on the DB level as you describe I think is too complex, because databse structure changed a lot from jbpm 3 to 4 and even more from 4 to 5. I wouldn't doubt you get that easily working, but would be very interessted if you did :-)
Cheers
Bernd