Registering devices with Intune for management and policy enforcement
@Shekar C, Thanks for posting in Q&A.
From your description, I know you want to migrate Devices from Entra Joined to Entra Hybrid Join.
As @Rahul Jindal [MVP] and @Marcin Policht mentioned, currently, there is no such way to perform migration without deregistering the joined devices, because it does need to disconnect the devices from Entra ID and then do Hybrid Entra ID join.
And if you want to do it, you can consider the method @Marcin Policht mentioned.
Hope it can help you.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.