There are no plans (at least in the medium-term) to provide such an upgrade function for the core war. As you are aware, due to the potentially destructive nature of a war upgrade, it should only be performed in a production environment as part of a well-managed process, which includes backing up the DB etc, informing users, etc. As DHIS 2 instances are highly configurable, it quickly becomes very complex to manage upgrade processes in a generic way.
If you purely mean upgrading the war file on the fly, you have options such as tomcat manager, for war file deployments through a web interface.
Apps, on the other hand, are a little different. From 2.32 onwards we plan to release apps independently of the core. This means that app enhancements and features can be released more frequently and you can choose when to take them. The apps will be deployed via an enhanced app store, and you will be able to upgrade to the latest compatible version of an app “on the fly” via App Management. You should also be able to “downgrade” apps too; for example, if some features in the new version are problematic.
So you will be able to “upgrade” within the (API/DB compatibility) constraints of the current core version.
I know this is not exactly what you are asking, but we hope that it makes it easier for users/admins to take advantage of new features.