@Kirsten287 You had the option to decline the revision request and maintain the notice period that's stipulated in the long-term cancellation policy. But since you accepted it, it's not considered a cancellation - just a change. You can't pull a random cancellation fee out of your butt just because it feels unfair.
You're definitely out the money in this case, though I hope you can get replacement bookings to earn it back. You might consider a shorter maximum stay, since Airbnb is not really designed for long-term stays anyway.