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I understand that you would like to convert the Entra registered to Entra joined device.
Microsoft Entra registered are personally owned devices. The users are not required to use a corporate account to log in to the device. The device is automatically Entra registered when a user logs on to an organizational application using their corporate identity.
To answer your question, as of now there is no option to convert the Entra registered to Entra joined device.
If you want to achieve your ask you need to remove and re-enroll the device.
Hope this helps. Do let us know if you any further queries by responding in the comments section.
Thanks,
Akhilesh.
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