As for my eperience upgrading, this post summarizes it. https://community.jboss.org/thread/200546
Another thing... when using the mapping on JPBMorm.xml I had to change date fields from type DATE to TIMESTAMP. First, because DATE makes you get only the ymd part of a date... which may not be very useful. Second, because doing so also fixed some issues (I don't really remember what the problem was anymore... but I would advise using TIMESTAMP instead to make sure)
Please notice that I didn't really migrate any data though... I just upgraded the jbpm version and deleted the old data from the database.