Assign Sub-Categories to Inventory

Go to Management > Assign Settings to Inventory and you will find an option to assign subcategories and subsets. 

Once the program comes up, you'll use the filter section to locate the products you want to change.  Then you'll select the new subcategory you want them to have.  Once selected, you'll click on Apply and it'll be done in a split second:

You'll do the same for the subset by using the filter to isolate the ones you want changed, and then selecting the new subset and clicking on Apply: