If you delete a Collection, all cards inside that collection are deleted. There is a simple way to transfer active cards from one collection to another, but to transfer archived cards from one collection to another, you must unarchive all of the cards, transfer them to another collection, and then archive them again.
It would be nice to be able to transfer archived cards from one collection to another without having to go through all of those steps. To have the ability to simply transfer an archived card to a new collection.