You have to migrate those "Manually". (Delete on prem, recreate in ExO)
You can use this script from Tim:
https://timmcmic.wordpress.com/2021/04/25/4116/
also some tips here:
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Dear Support,
I have found the answer for migrating an on-premise user email account to an O365 cloud account using this link - https://video2.skills-academy.com/en-us/answers/questions/1342373/migrate-the-on-premise-email-account-to-the-o365-e?comment=answer-1297398&page=1#comment-1344046. Could you please confirm if the same process will work for migrating a distribution group as well?
You have to migrate those "Manually". (Delete on prem, recreate in ExO)
You can use this script from Tim:
https://timmcmic.wordpress.com/2021/04/25/4116/
also some tips here: