THIS is why you book your flight through your department's agent --- you get a corporate or state rate, which is often nearly as low as your bargain fare. Then, if you need to change flights, you do not incur a penalty.
Lesson learned. Either eat the fare change or explain that you cannot meet at the appointed time, but can meet earlier.
From the realism department: sometimes these "corporate" fares are extremely high--I used to be able to fly out of DCA to home (ALB) for $200 to $250 when the fed gov't rate was consistently over $600. These differences are substantial, particularly when dealing with grad student budgets. Bottom line: the corporate rate is not always cheaper than the nonrefundable rate. I was proud to say I saved the taxpayers (who paid for my flights) over $15,000 by not buying the corporate welfare government rate.
Committees know this, and I believe will try to be accommodating.