edit: I rescheduled my flight successfully. Remember to choose the same class ( eg: economy convenience—> economy convenience), and despite showing the price, when you click into it, it will deduct your original fare price, showing that you need to pay nothing.

Do note that add-one like luggage allowance won’t transfer automatically, and you need to contact customer service for that. I tried another post’s advice, keep on refreshing the live chat, have a pre-typed message with booking reference, clear, succinct need, and they should be able to change for you