@Georgi Krushovski Thanks for posting in our Q&A.
Based on my understanding, Azure AD connect have synced the targert OU which includes local users to Azure AD when you deployed Hybrid Azure AD. You can use the user which is listed in Azure AD portal to do Azure AD join.
Based on my research, I find someone has shared some information about migrating from Hybrid Azure AD Join to Azure AD Join. Please refer to the following link:
https://learn.microsoft.com/en-us/answers/questions/500067/migration-from-hybrid-to-aad
Honestly, I haven't tried it and just a reference.
Hope it will give you some ideas.
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