How does SCCM Current Branch automatic client upgrade work

Once new client is promoted and deployed to the production machines based on the client upgrade settings, the client machines will upgrade to new client.
If we want to monitor the process as it happens, follow the below steps to see and monitor the progress. Some times it will help us to troubleshoot upgrade issues.

1. After deploying the client to the production using hierarchy settings, on the client machine run Machine policy Retrieval & Evaluation Cycle
2. The existing agent will detect the client version and start processing the upgrade request. Open ccmsetup.log and we see these initial entries;

3. Existing client will query for ccmsetup.exe and download ccmsetup.exe from assigned MP;

4. The machine policy will check for existence of Configuration manager client upgrade task, if doesn’t exist then Configuration manager client upgrade task will be created in scheduled task;

5. Now open the task scheduler and we can see Configuration manager client upgrade task. The newly created scheduled task will run on random timings.

6. For testing or immediate client install, Run the Configuration manager client upgrade task from task scheduler

When we ran the task scheduler, ccmsetup.log (C:\Windows\ccmsetup\Logs)will show ccmsetup is launched by windows task scheduler for client upgrade;

7. Once the setup is initiated, the scheduled task will be deleted from the Scheduled tasks

8. Wait for 5-10 min to complete the installation and check the ccmsetup.log file. 

    It should have ccmSetup is existing with return code 0