Orgs that successfully upgrade unlock new functionality and capabilities. As part of this new functionality, behavior changes slightly for a small portion of the features. Some of these experiences are new concepts that change when the org upgrades, and some appear when you configure the new functionality.
Changes for admins:
After the upgrade, end users see changes in the sign-in experience. Some of these experiences change immediately after the upgrade and others appear when your enable new features for the org.
Changes for end users:
To better help prepare your users, use the End User Adoption Toolkit so they know what to expect during and after the upgrade.