Only do this if you know there is a particular box available in your area. Grocery stores always must put older stock on the shelves first and if the box is new it may be waiting in line to get put on the shelf. You can use the extras to trade for others you need in your collection. If you wait to go to the store later to buy more, they may be gone. I carefully open BOTH ends of the box and remove the bag of cereal.
Some bags are spot glued to the side of the box, so be careful in pulling the two apart.
I store most of the empty boxes flat. You can buy these top loaders from dealers that advertise in card magazines. In quantity you can pay as little as 70 cents. For the boxes I want to display, after opening them, I fill the box up with little plastic peanuts usually used for packaging. Then I use a low temperature glue gun to carefully seal the box.