File Manager and Multimedia Manager will continue to be used on your site, so there is no pressing need to migrate files that are already in your system. Links to these files will continue to function. If you have a file in one of these locations that you want to use in Posts or Composer, migrate that file individually. Consider your move to Resources as an opportunity to clean house and focus on the future of your website, rather than carrying over years’ worth of baggage that may or may not be useful to you anymore.
For instance, you may have a folder in File Manager with important documents for parents at the beginning of each school year. Some of those documents are reused year after year, while the school handbook gets updated each year. Migrate your evergreen documents that you need for next year, but don’t import the entire folder with all the old handbooks you’ll never use again.