For those who haven’t upgraded, Apple’s support document outlines what you need to do: Make sure every device you plan to use the Home app on is running a supported operating system (“iOS 16.2, iPadOS 16.2, macOS 13.1, tvOS 16.2, or watchOS 9.2 or later”), open the Home app, and tap the update prompt. Apple says that some users “may be automatically upgraded to the latest version of Apple Home,” but it didn’t specify under what circumstances an automatic switch will happen.
Apple has never gone into explicit detail on just what the updated Home architecture does, but the additions the company has made to its Home (formerly HomeKit) ecosystem in the last couple of years generally work only with the updated architecture and app. According to Apple’s support page, that includes “guest access, robot vacuum cleaners, and Activity History in the Home app.”
For features that require a “home hub,” like sharing control of a home with other Apple users, receiving notifications, or adding accessories compatible with the Matter smart home standard, switching to the new architecture will also require you to have a HomePod (either full-sized or mini) or a modern Apple TV box on your network. The old version of Home/HomeKit allowed iPads to function as home hubs, but they can’t be used that way with the new architecture.