If your itinerary gets changed, they pay the difference...
If you’re massively delayed and the airline arranges alternate transportation with another carrier, they will cover all the expenses and extra fees the new airline might assess. So if there's only a first-class seat available, it's yours, and it won't cost you an extra penny. Pass the complimentary Champagne.... and, in that case, you get to keep your original ticket for later
That unused ticket for the delayed or canceled flight? It’s still good to use another time; think of it like an airline credit you got for your aggravation. If you’ve had it with that (expletive) airline and vowed never to fly them again, even for free -- you have principles, dammit! -- you can also request an “involuntary refund” for the flight from which you were bumped.
One point of warning: There HAVE been instances of airlines trying to cancel your original ticket onsite, and confused passengers often assume this is normal procedure. It’s not. Politely tell the reservations agent you do not want to cancel the existing reservation.
Non-refundable tickets CAN become refundable
If a flight is severely delayed (generally over two hours) or canceled or if there’s a schedule change in advance or a route change (like a nonstop flight changing to a flight with connections), you can get a full refund on a non-refundable fare. Don't let them push you around. When the airlines at fault, it owes you money.
Your additional fees are refundable, too
Though common decency would dictate that the money you paid to check your bag, get some extra legroom, or board early would also be refunded in the case of you getting bumped or severely delayed, airlines don’t always offer it up. Make sure to mention the fees you paid when negotiating any compensation or refund. If you’re nice, and your agent isn’t having a bad day, they’ll sometimes give you that stuff gratis on your rescheduled flight as a gesture of goodwill. Again, the key words here are “if you’re nice.” Be nice.