to be fair if you have a production cluster of elasticsearch or postgres you can't just upgrade the DATABASE technology without interrupting the cluster. BUT you can upgrade the OS from CentOS 5 to 7 without a problem. And the Elasticsearch/Postgres Cluster would still be fine
Depends on a lot of things... Often you'll be using the OS provided postgres package, so upgrading from 5 to 7 will probably also involve a big database version update. Do you have any associated tooling or services? They might need updates to work with newer libraries or software updates, such as Python 2.5 to 2.8. Etc...
Over the last couple decades, I've done something around 100 of these sorts of upgrades for clients of a managed hosting service I used to run, and they were rarely simple OS upgrades. Lots of planning and testing went into them.
That's the sort of upgrade this article seems to be about, I'm not sure if it's because SQL Server is tightly integrated with some OS-provided services that were incompatible, but it was certainly more involved than a base OS running only the database and just installing the same database on the new OS.
