I used to work on a company that had MongoDB as the main database. Leaving a lot of criticism aside, the replicaset model for Mongo made the upgrades much easier than the ones in other type of databses.
While that’s true, managed services on eg AWS provide hot replica’s as well, which you can use to upgrade the database and do a failover to the new version.
We actually migrated from vanilla Postgres to Aurora that way with minimal risk / downtime, it was a really smooth process.
