Just found this:

Compensation for Involuntary Denied Boarding

If you are denied boarding involuntarily, you are entitled to a payment of “denied boarding
compensation” from the airline unless:

(1) you have not fully complied with the airline’s
ticketing, check-in and reconfirmation requirements, or you are not acceptable for
transportation under the airline’s usual rules and practices; or

(2) you are denied boarding
because the flight is canceled; or

(3) you are denied boarding because a smaller capacity
aircraft was substituted for safety or operational reasons; or

(4) on a flight operated with an
aircraft having 60 or fewer seats, you are denied boarding due to safety-related
weight/balance restrictions that limit payload; or

(5) you are offered accommodations in a
section of the aircraft other than specified in your ticket, at no extra charge (a passenger
seated in a section for which a lower fare is charged must be given an appropriate refund); or

(6) the airline is able to place you on another flight or flights that are planned to reach your
next stopover or final destination within one hour of the planned arrival time of your original
flight.