Hello Touil,
Can't see any script that install the database files. In build.xml you can only see the start.db.files.
What I am going to do is to Backup/export the JBPM and Task tables in Mysql. Then, clean everything and install Jboss AS7 with related deployments and the task service and run them.
I will notify you if this gonna work or in case I found a better approach.
Regards,