Originally Posted by
mvoight
If store clerk gives you change for $20 when you gave them $10, is it not stealing if you keep it?
Stealing is taking something, not keeping something given to you. There are cases when you shouldn't keep something, but that is a matter of honesty, not theft.
A better analogy to OP's flight rebooking is: what if I order california rolls, but upon delivery, I receive rainbow rolls, which are much more expensive. This happened to me yesterday. I just went ahead and ate the rainbow rolls. Did I steal from the restaurant? Was I dishonest?