Where’s the risk on gradual enhancements vs redesigning the whole app?

I'm working on a legacy software built on Java 6, the design is old and the components look like they're from Windows 98. We're introducing a new big feature and one of the requirements is to design the feature with more of 2020 trendy look and feel. (to show that change is coming)

My concern is that this might damage the consistency and the user experience if we're launching a feature that looks completely different than everything else in the application. Should we design the new feature with the components we currently have on the current app? This way we focus on the value more than aesthetics and later this year we can redesign everything once and for all