Not everybody using Cinemachine needs to upgrade. But if you’re starting a project in Unity Editor 2023.2, you’ll have Cinemachine 3 by default.
Please note: Before upgrading to Cinemachine 3, check if you have custom scripts that make significant use of the Cinemachine API. If so, it may be better to stick with previous versions. And, although we provide a tool to convert project data, there is currently no automated way to migrate code. The Unity API updater can’t take care of all the API changes, so this will require a manual upgrade.
If you are using Cinemachine “out of the box” without any custom scripting, the upgrade process will be relatively easy.
Source: Unity Technologies Blog