I had the same issue and very little to do with CX but QR. QR has very strict married segment policy. If seats are still available DFW-DOH, then you need to cancel your existing seat and rebook it. This can be done on the same booking reference so you only pay the reissue charge rather than the redeposit charge (difference is only 20usd or 2K asia miles). My suggestion is go online on AM website, search DFW-MLE, and if it shows up book it there, secure your seat and then cancel the economy ticket. (they will charge you reissue fee for upgrade of ticket of 10k asia miles or 100usd). This way your seat is secured throughout on a new ticket, then just cancel the exisiting one, and that will cost 120usd to 12k asia miles so difference is not much. Just had to do that for my BF's ticket from AMM-NBO, where AMM-DOH was orignally in Eco but then first opened up.