I have had similar problems and QR have been able to find good solutions.
I would recommend you to:
1: Search and find options with other airlines on the same day (based on your profile and comments I assume your flight is from the UK to Hanoi or Da Nang). It is probably easier to be moved to OneWorld airlines but not sure about this (the two times this has happened to me I was rebooked to Finnair).
2: Call Qatar airways and tell them that you have looked at options and ask them to change your out bound.
My experience is that the customer support is bad at finding solutions, but if you present them with something reasonable they can normally make it happen.
Hope you manage to find a good solution