We’ve often dealt with that same issue — every time we design a new feature or dashboard, trying to balance between consistency with the legacy designs and not feeling dated or providing lower quality UX. We’re also slowly updating all the legacy stuff, so over time we’ve shifted from designing in the old UI scheme to a hybrid one, and now designing fully in an updated scheme. The key for us has been using parallel design structures and elements in the new UI to make it feasible to do all this incrementally.