You could buy a discount southbound ticket and a changeable northbound ticket to minimize the high cost of a flexible ticket. Alternatively, if you're a student, websites like www.studentuniverse.com have some tickets with more flexible rules than regular airline tickets. (You'd need to check with them for the precise details, there is still a change fee, but it may be less.) Finally, if you were to use SkyMiles you could opt for an unrestricted award ticket to reduce costs if you need to change.