You can change Card Categories in bulk from a search in your Project Library. 

Hit the bookcase icon on the far left of your screen to access the Project Library. 

Select "Cards" and filter your search by card category, fields, or type a keyword.

When you have all the cards you wish to add, select onethen hit the double ticks that appear in the bulk actions toolbar to select all.

Choose the "Change Card Category" option and select the category from the dropdown menu and save to update.

Note: This will change all of your cards to the new category, and if the new category doesn't have all of the fields in your existing card, these values may be lost.