@madeofsteel This would get a little more complicated. Add.js only adds whats there, to change or update you would need to use change.js, which if im not mistaken can not add any products at all, only change whats already there.
So you would need a developer to dig into it more, Adding an IF Statement to check if the product is already in the cart. If it is, then use Change.js, otherwise use add.js.
add.js should work fine if they are adding different sizes, since it uses variant ID.