Which is a curious paradox that they allow this (likely because most people don't take advantage of refaring on the whole). By opening up lower fare buckets, they're lowering the cost of the flight to help drive new bookings, but by refaring to a lower bucket you take up some of that space (potentially taking the last space in the lower bucket which opened), thus driving the price back towards where it was in the higher buckets, before they lowered it to drive new bookings.