The first applications to get a new look include Calculator, Groove Music, Mail, Voice Recorder, Alarms & Clock, Movies & TV, and Calendar, according with The Verge, and more will follow.
Old Windows icons have been around for years, even when Microsoft began to adopt the minimalist pictograms of the design era of the "Metro", did not redesign all the Windows icons, leaving us with the old ones.
The Fluent Design was first presented at its 2017 Build conference Microsoft, showing us an interesting new stylistic direction of the company. Combines all elements of lighting, motion, depth, material and scale and creates customizable elements that work on various devices (computers, phones, TVs, VR).
Her latest promotional video Microsoft for the new integrated Office application gives you a good idea of the aesthetic direction in which the company is located.