Two choices.... most correct: log in to the shell and use 'mv' or 'cp'
IMO cheesy but easy (assuming both domains are using the same USER on the server... log into the dreamhost panel and edit the domain def.org and set the webpath to /home/USERNAME/abc.org
Be away you might have issues if the ground up php apps were built with anything hardcoded to a url or domain for example. If it were me making the change I would use ssh and check first using a 'grep -e "abc.org" *' (without the single ' ) to see if any files needed to be modified.
third choice use an ftp client isntalled on your machine such as winSCP and rename the directories
abc.org -> temp
def.org -> abc.ord
temp -> def.org