Guillaume, why don't you consider upgrading to 3.2? It is a much smaller jump, you get some new features and  improved stability and performance. The database models are sufficiently compatible as to allow for data migration. The API did not change much either. The 3.2 branch still receives periodic updates and you can even get dedicated support for it. Once jBPM 5 is delivered, there will be a complete migration path from version 3.

On the other hand, do note that there is a process convertion tool from jPDL 3 to 4. The scope is limited to the process definition; no attempt at data migration is made. Given these comments you should be able to decide what is best for your situation.