Hidden cells also get changed when I try to change a filtered cell

Solution 1:

And alternative to dragging the contents of a cell, you can Copy/Paste:

  1. Copy the first cell containing South.
  2. Select the cells of the 15 products to be changed.
  3. Hit Ctrl+K (Or F5) then click Special...
  4. Select "Visible cells only".
  5. Paste with Ctrl+V

Note: And alternate to steps 3 & 4 is to hit Alt+; (Hold down Alt when pressing ;).