@Dinesh Loganathan, Thanks for posting in Q&A.
From your description, I know you are looking for a way to migrate MDE to Intune.
Based on my research, MDE is not an MDM provider, therefore, you can migrate to Intune with an easier way.
As for the problem: what will happen if I enable automatic Intune enrollment to the old/existing Windows 10 devices that are already onboarded to MDE have a synthetic registration in Intune and are Managed by status MDE?
The devices will not process policies for Defender for Endpoint security settings management, which is a feature that allows you to deploy endpoint security policies from Intune to manage the Defender security settings on the devices without enrolling them with Intune.
Hope this can help you.
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