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:
UC Slideshow
UC Slideshow does not work with the newer Cornerstone based themes. You can use the same images, but you'll have to load them up with Meta Slider.
Navigation
The Primary Navigation menu needs to be set in Appearance > Menu; in Hale there was an option to set it in Appearance > Customize.
Bootstrap
If you had pages built with Bootstrap, you'll have to redo them because the new themes use Bootstrap 3.
User Profiles
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.