You do not need to rebuild the app, changing the -ds.xml file and restarting is all you need to do.
I am not sure about the second question, but it is almost certainly controlled by build.properties somewhere.
For the first question as Stuart mentioned all you need to is change the xxx-ds.xml in deploy folder and restart your jboss.
About your second question, it is controlled by an ant property. Default is dev. Look at line 7 and 8 in build.xml
This will rename your persistence-prod.xml to persistence.xml.
Yes, thanks all set.