Had a similar experience with Turkish. Free flight change but the website didn’t work and every time I called them I got insane price differences that were almost the same as a full flight ticket. I ended up going to the airport sales office and doing it there (still did not come cheap)

Qatar also offers a “5% premium” when you request a voucher instead of a refund. But hey the premium is on the price before some base fees, so the voucher total is almost exactly what you paid, not 105% as you’d expect.