Managed to speak/chat with 4 Expedia-agents today. None of them were unable to change my flights, they came with different reasons (like: "AMS-BUD has to be operated by Delta!" & "We don't have any availability after September"). Was going to call Delta to ask whether they could take over the ticket against a fee. To my surprise, the UK agent was super helpful and accommodating. Took over the ticket without charge, made a new PNR, put in all my flights for Jan 2022, checked the fare rules (I class, HIP) & sent the new itin for confirmation to the pricing desk. He said this normally would be unneccary, but since I had an invol change last autumn, a manual check is needed now. Promised me I would hear from Delta within 24 hours (hopefully a new e-ticket).