Changing the definition of a process that already have running instances (even if they are persisted) could be risky. There is no simple answer to your question. The behavior of your processes, once resumed from the database, will vary and it will depend on what chages did you do to the process definition (added/removed/modified nodes) and where each of the process instances was when it was persisted (after/before a changed node or even in the changed node itself).
There is a section in jBPM5 documentation explaining a little bit the options you have.