You might want to investigate the custom approach this merchant developed on their own:
http://muvenum.com/buy-now/
If you Add to Cart an item there, you will see they devised a way to add an upgrade item inside the cart itself, and if you add that upgrade item and then remove the original, the upgrade item gets removed as well. We cannot provide further assistance with this, as we don't actually know quite what they did or how and weren't involved in that anyway, so any questions should be directed to that merchant if they're willing to help.