@SUNIL KUMAR This has been a common movement requirement now. many other companies are also trying to shift towards complete Azure AD Joined state.
Although the move is not a very straight forward or MS has not provided any migration path for same.
The device state (Azure AD registered and Azure AD Joined ) were both for a different scenario, where AAD registered were for personal devices for corporate resource access and Azure AD joined for corporate devices to utilize more features like SSO and device management.
There are many ways to achieve this, it all depends on which route would be easier for you.
1) Azure AD Joined via Autopilot which needs Intune
This requires you to fetch the hardware hash from all devices and upload them into Intune, followed by Autopilot profile to be targeted to all devices.
This is a easier way for bulk enrollment and management. Make sure that you will have to clean up the Azure AD registered device entries from Azure AD portal.
2) Involve users to perform AAD Join
If you have small group who can perform some extra steps to convert their devices from registered to joined, you can disconnect the devices from settings, work and School account, followed by deleting the entry from Azure portal.
Then go to settings - Work or school account - Join to Azure AD option.
The methods depend upon which route you want to choose. Let me know if you have any questions.
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