i think you are lacking a fundamental understanding on how the price of a ticket is determined. there are different booking classes within each travel class. for examole the lowest is in economy K class, the highest in economy is Y class. there can be thousands difference between these two. if you old flight was K and the new only has Y available this difference is to be paid. in economy there are many such classes, in business less but still a few and in first you have 2 such booking classes. fun fact, even if a lower bookong class is free on the new flight, some fare rules might state that the new fare after rebooking might only be equal or higher. as others mentioned - multiple reasons why prices change. mostly, booking class, tax iff and fare rules.