Yes you can = but the new fare applies. ie if you issued a OWE ticket last year before the fare increase, if you want to change from a 3 con to 4 con ticket, you will just pay for the fare diff between your OLD 3 con fare and the NEW 4 con fare!

However, please note that once you have returned to your originating Continent, you cannot then 'add' the new continent to your ticket!