Migrate content with user progressively

Okinawa Institute of Science and Technology Graduate University (OIST) built a Drupal 7 website with Organic Groups to accommodate all kinds of sub-sites in the Campus, including administrative devisions, committees, research units, student clubs, etc.
The website has been built since 2012 and uses Bootstrap 3 as base CSS framework. Bootstrap 3 is used by the administrator of the website to build pages, it is also being used by all members in the campus to create their own content. There are more than 30,000 pages in the website.
We are going to migrate and upgrade the Drupal site including the base CSS framework Bootstrap. All user generated contents will need to be compatible to Bootstrap 4 or 5 when the site gets migrated.
Having all content author work together to migrate the content is a key to the migration. We're exploring a way to work with all content editor, and provide them a progressive and smooth path to make the content compatible to the new CSS framework.
We would like to share our approach by the time of the DrupalCon and welcome for all kinds of suggestions from experiences.