You can ask for a return for the difference in fares if Qatar Airways changed your seat class without giving you a reason.
Start by calling +974 4023 0000 or going to their feedback page (https://www.qatarairways.com/en/help.html) to talk to customer service.
Give your order number, flight information, and proof that you were downgraded, like your original ticket and boarding pass.
International rules, such as the Montreal Convention or EU261/2004, say that airlines have to make up for downgrades by refunding 30 to 75% of the ticket price for the affected section.
Make it clear what you want, like a refund for the difference in fares or some other kind of pay.
If your request is turned down or not dealt with, take it to the next level by complaining to Xolvie.
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Especially with an international airline, you have quite high expectations? You can basically throw all of those out the door when dealing with these folks. In this light, imagine: I had heard great things about Qatar Airways, but I swear that was the most unpleasant and claustrophobic trip I've eve...