Fare rules - or rules for changing tickets?

Finnair sells "changeable" tickets for "fare difference". What they don't disclose is what they mean by "fare difference". You would think that this is a difference between what you paid for your ticket and what you would need to pay for the ticket you are trying to change into. But that is absolutely not the case: they have a hidden rule that you can only change into the same or higher fare class or ticket category - even if a lower fare class is available. IMO this is creating a contract that implies one thing, and then not honoring that contract. The only other airline I have experience with is Alaska - and with Alaska, they do changes the way you would expect the fare difference, allowing you to go down in fare class. Moreover, Alaska gives you travel credit if your new fare is lower. I came in with this expectation to Finnair, and was very disappointed. And not only disappointed in the policy, but also incensed that this policy is not disclosed up front.