I would like the option to add a 1-click upgrade to a funnel. With this option, the upgrade product would REPLACE the original product instead of adding it to the order.
Here's an example: A customer begins the checkout progress for our monthly SaaS plan (product 1), but then they see an offer to upgrade to our annual plan (product 2). They click "YES" to accept the offer to upgrade.
The system should REPLACE product 1 and they would only get charged for product 2.