I did call back, and the second agent I spoke with also initially said there'd probably be a change fee. When I questioned the agent and mentioned my previous experience with rollovers, the agent brought up the fare rules and said I would be entitled to the fare difference, and it has been credited to my original form of payment ($43, as expected).

This will teach me to never book winter fares in August, even if I can get the systemwides to clear (surprisingly, even today, there are routings still available that I could use systemwides on the same days of travel, if I was looking to make an initial booking).

Now, it will be my luck the fare will drop again, and I'm not sure I'd have the nerve to call back and ask for another credit.