Is it possible to swap two cells content in numbers?
If you don't want to use other cells temporarily, then do the following, which assumes equal size ranges:


Select the first range of cells.
Press ⌘C to copy the first range of cells to the clipboard.
Select the second range of cells.
Drag and drop the second range of cells onto the first range of cells.
Select the range of cells the second range of cells originally occupied.
Press ⌘V to paste the first range of cells from the clipboard.


Now the two ranges of cells have been swapped.

Note: Both ranges do not necessarily need to be the exact same size, however there has to be an appropriate amount of cells in the area of the ranges being swapped. Use your best judgement.

By the way, this works because the drag and drop clipboard is different then the general clipboard. So the first range of cells just sits there waiting to be pasted as the drag and drop operation didn't disturb it.