So what a customer needs to do is take a screenshot or get a quote of the exact same flights but in discount economy and premium economy when booking their business class fare. They need to get this for the same date/time period of booking, for an accurate reflection of what the cost would have been – or what the difference would have been.
I guess the question then becomes, if you have that information, how likely is a customer to be able to force an airline into refunding that difference, and not just the difference between full fare Y on the night of the flight, which would obviously be much smaller.
For example, say I book an $8k business class flight (return) but at the same time I look at what a sale/discount economy fare is and screenshot a fare of $999; the difference is $7,001. On the night of travel they oversell and downgrade me, but economy class at the time is selling for $5k, so they only want to refund $3k to me. If I have the evidence to prove that at the time of booking the difference was more than double what they're offering me now, what are my chances of winning?
This seems logically the fairest outcome – difference between fare paid at time of booking, but we all know this is not how an airline operates. So what are the odds of being successful and why is the onus on the customer to screenshot and provide proof of the fare difference at time of booking, and not the airline?