I updated this blog to WordPress 5 yesterday.
A couple notes:
1. When I updated, even though the Classic Editor was installed and active in 4.9, the update disabled the plugin! Grrrrrr. So, first thing I had to do was go to Plugins and re-enable it.
2. There are new settings under Settings > Writing: “Default editor for all users” and “Allow users to switch editors”. Set them to the “Classic” and “No”, as shown. If you’re using Advanced Custom Fields you’ll get the white screen of death if you don’t. (Update: Updating the Classic Editor plugin to version 1.1 should clear up the ACF compatibility issue.)
3. That said, even with the “Don’t allow users to switch” setting, the posts page will ignore it and offer users the option to edit posts in either editor, so (at least for now) it’s not a bullet-proof prevention to disallow what’s now called the “Block Editor”. (Update: again, updating to Classic Editor 1.1 seems to have cleared this problem as well.)