AS has been doing this for awhile, the offers are always laughable. At least with AS, they don't appear to be about being oversold - more like trying to move people off flights they're getting higher fares for easily, in the hopes they can continue to do so? I've taken them up on it exactly once, when it was offering a later flight that was more convenient for us. The amounts they offer are always nominal and would never convince us to switch if it weren't legitimately more convenient.