It appears that door upgrades work a little differently than standard furniture upgrades. Or, to be accurate, maybe they NEED to work differently. Once a door has been placed, it cannot be picked up, nor can it be moved to a different floor. So, if you purchase an upgrade for a door on the first floor, it is "locked" to that door and there is no way to "move" it to the 2nd or 3rd floors.
In my decorating, I realized that the door I had placed on the 1st floor would look better on the 2nd floor and visa versa. Both upgrades were purchased using credits. I thought exchanging them would be a simple matter. I was wrong. The only way to make the change would be to spend more credits to upgrade both doors.