Ananth,
1. No, the jBPM5 API is not compatible with the jBPM3 (or 4) API. Running jBPM3 processes requires the jBPM3 runtime, and I don't think any migration stratgey for this (like the common base class you propose) is foreseen by anyone - at this point.
As Bernd points out, migrating running process instances is going to be even harder than migrating process definitions. This is already the case when upgrading between minor versions (e.g. 3.2.2 -> 3.2.3), so between major versions (let alone a complete overhaul such as jBPM5) will be pretty impressive!
2. Migrating data will also be hard, partly due to the aforementioned API changes. But when you try something like that, be sure to separate you business data from the process execution data. That way, you should be able to make any transition as smooth as possible.
Like Bernd says: we're very interested about all migrations - successful or not. We're very much committed to making the transition from jBPM3 (the current productized version) to jBPM5 (the next version that will be part of the SOA-P) possible.
Regards,
Maurice