Thanks!

Is it the difference between the biz fare I paid at booking and whatever the prevailing economy fare was at the time that I booked? Or what the economy fare was when I was switched to economy an hour before my connecting flight left? Is there an easy way to see what past fares would have been? I didn’t take not of what the economy fare was when I booked.

I guess one way would be to look up a future business flight that cost roughly the same as what I paid for my flight, and then check the fair differential between the two classes, assuming it would be roughly the same.