Yes, permissions will be retained .
You may need to re-do SEND AS perms if they have any of those however.
https://learn.microsoft.com/en-us/exchange/permissions#mailbox-permissions-and-capabilities-not-supported-in-hybrid-environments
https://learn.microsoft.com/en-us/exchange/troubleshoot/send-emails/overview-delegation-office-365-hybrid
From what I have seen, if you are using the latest version of AADConnect and Perms are set now, they will continue to work after you move the mailbox to 365.