A lot of readers (intentionally?) get this wrong though and show entries as new/unread if there are changes to the content.

Of course a lot of feeds also get this wrong and change the GUIDs for existing entries once in a while which results in strictly compliant readers showing you the entire feed history as new. Really annoying.

My system updates¹ the entry and marks the changes in the first case. What is right and what is wrong depends on what you want, I think.

The latter is annoying, I agree.

¹ It is an NNTP interface so the article is superseded; https://feedbase.org/about/ - if you don't want to see updates, you can configure your newsreader to skip supersedes.