I just had to turn a YVR-YYZ-YVR into a YVR-YYZ-NYC-YVR. Not only had to pay the change fee, also had to upgrade the first sector from an L to an H.
Notwithstanding getting hit for a higher sector fare on the first leg--which was completely unchanged from my L booking--they would not budge on also charging me the change fee.
Ah well. That's what I get for not booking it properly in the first place. Better than paying $1200 for a same day YYZ-NYC-YYZ.