If they are really overbooked, it has worked for me in similar cases to get a simple reroute. Say you will accept the downgrade but ask that they place you at LH flight tomorrow (plus hotel plus some compensation) or the AF flight. This way you can also ensure double dipping as you will still be entitled to your original routing credit. If they are heavily overbooked, they should be happy to do (refer to future flights and that you may as well rebook those).