use jbpm's persistent config file to do this.
maybe the file name is hibernate.cfg.xml in jbpm package.
I think the idea here is that existing process's follow the process they had defined when they started. Thus you need to keep the original copy.
I heard on the jbpm forum that you can archive these off using the jbpm console... maybe once they have all closed.
see this link
I hope this can help you