Here's Rule 5A from AS' Contract of Carriage:

"Alaska does not guarantee allocation of any particular space in the aircraft. Seat assignments, regardless of class of service, are not guaranteed and are subject to change without notice. Alaska reserves the right to reseat a Passenger for any reason, including from a seat for which an additional fee has been paid, and if a Passenger is improperly or erroneously upgraded to a different class of service. If a Passenger is removed from a seat for which a fee has been paid, and the Passenger is not re-accommodated in the same class of seat or a seat of equal or greater value, or if a Passenger is downgraded from a class of service and is not re-accommodated in a seat of an equal or greater class of service for which a fee has been paid, the Passenger may be eligible for a refund in accordance with Rule 17."