You pay for a seat in a certain cabin (economy, economy plus or equivalent, business, or first). You do not pay for a specific seat. So if you buy an economy ticket, you are entitle to any seat in economy class and can be moved with no compensation whatsoever. If, for some reason, you book a seat in business class and are bumped down to economy (oversold flight or change of equipment), the airline would owe you a refund for the difference in the cost for class of service and a big apology, usually in the form of FF miles and/or a credit voucher.