“We had an overview meeting last Summer and we have those every year now where they give us a pitch for the season. We knew about the mirror universe because last year. We knew we were going to jump into the future because of Michelle Yeoh, and how she was getting out of our world and into the ‘Section 31’ world which is the next series, so it was an early conversation,” Phillips tells Variety.
https://variety.com/2020/artisans/features/star-trek-discovery-mirror-universe-costumes-1234850581/
https://variety.com/2020/artisans/features/star-trek-discovery-mirror-universe-costumes-1234850581/