We change URLs every day, for lots of different reasons—for example if one article is mostly just copying from another source, or if users suggest a better URL. When we do that we nearly always post something saying what we did, and including the previous link:
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
https://news.ycombinator.com/item?id=21378471
I remember commenting on a different article but the one that it points to now is a different article: https://news.ycombinator.com/item?id=21384151
Normally when we do that we explain so in the thread, like this: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que.... But this one happened overnight when no one who is public as a moderator was awake. That's a flaw in the current moderation system.