down gaging to smaller ac types has the same effect as an auction. Price/seat rises increasing yield. Moving to the 320 has the same effect, pax will pay more for the same seat as supply is reduced. The mix allows shifting ac types as individual markets begin to soften or become more robust.
IOW it allow the company to get price leverage on the micro level.
The cost to attain this leverage has to be weighed against the cost of displacements but that is what accountants do.