A launchpad exists so the team can learn from it, and learning needs something to compare against. Nielsen Norman Group's guide to benchmarking describes measuring an experience against a meaningful reference, and an earlier version of the same product is one of the references it names. For a launchpad, that earlier version is the site you're about to replace.
So take the baseline before the switch, on the pages you plan to rebuild, while they are still live. Bounce rate, exit rate and conversion rate are the measures Lean Labs' article watches on a launchpad. The method's page adds heatmaps and session recording from launch day.
A baseline tells you whether the numbers moved. It doesn't isolate the reason, because a launch changes copy, design, page structure and often URLs together, and traffic can shift over the same weeks. Where a decision rests on one change, a controlled comparison during the improvement stage is one way to estimate its effect. We list the measures worth keeping in what metrics to track in growth-driven design.