I've setup a music download section within ZC that gives the customer the option of purchasing the full CD as a ZIP file or purchasing track-by-track.

I've set the Mix/Max/Units to 1, 1, 1 for the entire item

The problem I am having during testing is trying to update (ie add/remove tracks) from something that is already in the cart. For example, try going here and adding the full CD plus a few tracks. Then select the item from the cart page and the product info will re-load showing the previously selected attributes/downloads. Because the max quantity is 1, any attempt to modify the selection will result in an error when the product is re-added to the cart to reflect the updated selection changes.

If max quanitity is >1 then it just creates a worse scenario allowing people to purchase multiple quanitities of downloads.

How can I set it up so that the changes just overwrites what was already in the cart?