I think I have it. He made a test site. With permanent link structure:
year / month / day / postnombre.
Published a publication. Then he changed the permanent links to postID / post-name. Tested with looking at network statistics from the browser. This is what I got, a 301 redirect, after trying example.com/2018/11/04/hello-world/ from my browser:
I tried something else. Using different permanent link structures, I tried to make two publications with the same posterior name (the part of the link where the subsequent name is found). WordPress will not allow it.
My conclusion is that post-name and postID are unique. This would explain why this automatic 301 redirection works, and why having a flat directory structure on example.com/post-name does not perform as slowly as it was before WordPress 3.3. (I think) the version.
Another interesting thing is that, when using the permanent link structure of postID / post-name, I can enter a browser: example.com/postID (with or without a forward slash) and the browser goes to example.com/postID/ ( with a trail) bar (slash) directly, showing the contents of example.com/postID/post-name.
So this seems to be well resolved, there is no need to worry about manual configuration of 301 redirects, either with .htaccess entries (my planned change would have required manual redirection for each publication, since I am adding a unique ID in the structure of the link). No need for add-ons too.
What you need to do is change all the internal links: WordPress does not change them automatically. It does 301 redirects, but the old links remain. That is a nuisance.