Live is not the finish line. Immediately after launch is the best time to start tuning the site to maximise commercial yield.
Following a user-centred design approach will reduce risk and uncertainty during design and give you a much greater likelihood of achieving your business goals immediately after launch. But inevitably there will be aspects of the new experience which need tuning: you may be under-performing against plan on some metrics and over-performing on others.
The period immediately after launch is a great time to work on this: a UCD project will have brought you closer to your customers and provide you with insights and ideas which you can use as the basis of tests to improve performance.
We normally recommend developing a test plan during the Build phase. That means you are ready to roll straight into optimisation immediately that you are live, stable and have a good handle on post-launch analytics. This might include A/B tests or multivariate testing or a further round of user research to review your new experience in the live context of your marketplace.