Originally Posted by rylan

I don't think you'll find a rule unless it is somewhere vaguely referred to in the contract of carriage. It is normal procedure to keep pax in the same fare basis and class as originally booked if a reticketing is needed due to a schedule change.

If there is a last minute rebook due to irops then an agent may just grab any bucket quickly instead of preserving the previous one either in the interest of time or due to being lazy.