i get that. However anytime you change outbound you must reprice all flights to current prices. The flights you are on are sold out. Thus you cant keep the same flight. No different than trying to change to a flight that is sold out.

Now they could've cancelled your reservation. Let the inventory go back for sale and then reprice it. However theres no guarantee all the seats would be available for sale again. If flight had overbooked, the inventory might not have been placed for sale.

I know it seems dumb, but it's how airlines price.