Do not do this!
It is the contents of a variable that should change, not the name of the variable itself. An example in real life, you hardly ever need to buy a new box if you buy new cookies. In matlab you can use indexing in your list to get a specific element:
MyBoxes = [10 11 12 13] ; % list of cookie types in each box
BoxToUse = 3
CurrentCookie = MyBoxes(BoxToUse)