KPI should be established at the point of inception. In working with the client, we want to understand the "why now" of the product request. Additionally, it's important to understand what impact the client is expecting and trace the KPI backwards from there. Specifically for this example, I would measure the number of transactions, the user behavior funnels, and validate that our key personas were funneled to the functionality appropriately so we can validate that they do in fact want to use this functionality. That said, we also need to very closely monitor support and sales channels since both can be clear indicators of expectations as well. Change management would be considered with this groups before releasing the feature into production.