Thank you for reaching out to us.
Usually, exchange hybrid option is selected when you have few mailboxes on-premise and few in cloud. When this option is selected in AD connect, it activates few additional attributes which will be written back from Azure AD to on-premise.
Using these attributes you can control few features of exchange online from on-premises environment.
Since you have already migrated all mailboxes to cloud, you don’t need to enable this feature. If you want to manage some features of exchange online from on-premise, then you can enable this feature and let few exchange attributes written back to on-premise.
Below is the article which talks about the attributes which get’s written back to on-premise.
https://learn.microsoft.com/en-us/azure/active-directory/hybrid/reference-connect-sync-attributes-synchronized#exchange-hybrid-writeback
There is no impact if you enable this option now in AD connect.
Let us know if you have any further questions.
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