So what's different if you're converting a Hale or Prudence site to Sherman, Hale 2015, or Prudence 2018? Here's a few quick points:
The Primary Navigation menu needs to be set in Appearance > Menu; in Hale there was an option to set it in Appearance > Customize.
If you had pages built with Bootstrap, you'll have to redo them because the new themes use Bootstrap 3.
User Profile Pages will need to have their page template reassigned. Go to pages, and use the checkboxes and the Bulk Action > Edit feature to do many of them at once.