How to move items to another category
The action to move items only is a little tricky as the term I've choosen (secondary categories) is not accurate. I should be called in fact "All categories" because it includes the primary category also.
So if you wish to move only your items from categoryA to categoryB (without having them in both categories), you have to choose the "move only" option and select "no" for keeping subcategories.
Then choose the categoryB in BOTH list.