When changing the version of an existing cluster, either a major or a minor upgrade is performed. The difference is that a minor upgrade takes you from 2.2 to 2.3, for example, and requires no downtime as a rolling upgrade is performed. A major upgrade takes you from from 2.3 to 5.0, for example, and requires a full cluster restart as part of the upgrade process.
Bug fix releases also require no downtime when upgrading. A bug fix release takes you from 2.3.4 to 2.3.5, for example.