When you enable co-management, Configuration Manager is still the management authority for all workloads. So you continue to manage your devices the same way. If you don't switch any workload to Intune, all of the Configuration Manager settings and apps continue to work the same as before you enabled co-management.
Enabling co-management is completely transparent to the end user. There's no reboot, no agent installation, no interruption, and no user notification.
As an administrator, you can see co-managed devices in the Microsoft Intune admin center. You can do remote actions from the admin center, and can benefit from insights with Endpoint analytics.
Co-managed devices could be transformed to Autopilot-provisioned devices next time the device is reset.
Consider using the Company Portal. It's the cross-platform app portal experience for the Microsoft Intune family of products. By configuring co-managed devices to also use the Company Portal, you can provide a consistent user experience on all devices. Your users should get familiar with Company Portal as it will provide the integrated experience going forward. For more information, see Use the Company Portal app on co-managed devices.