I don't know where you are going in Toolkit, but when I select Edit | Inventory | Category Changes, I don't see any mention of item numbers.

Same result for importing using File | Import | Inventory | Category Changes.

Try using one of those functions. I don't think you'll need the import, since you are only changing one category.

A warning that it will change ALL items and all transactions assigned to that category, so if you only want to change selected items, Toolkit is not the way to do it.

Steve