Migrate from Exchange 2016 to Exchange SE

Hi @Ed 
Thanks for reaching out to the Microsoft Q&A forum.   
Based on your description, I understand that you are seeking guidance on upgrading from Exchange Server 2016 to Exchange Server SE in your hybrid environment. 

As a forum moderator, I don’t have a dedicated testing environment to replicate exact user-specific scenarios. That said, my response is primarily supported by official Microsoft documentation, and I’m happy to share what I’ve discovered that I believe may help provide more context for your situation 

The image and this article Upgrading your organization from current versions to Exchange Server SE | Microsoft Community Hub indicate that to upgrade from Exchange Server 2016 to Exchange Server SE, you have two options: 
+Legacy upgrade to Exchange Server 2019 CU15, followed by an in-place upgrade to Exchange Server SE 
+Direct legacy upgrade to Exchange Server SE RTM/CU1  From my research, Microsoft recommends the in-place upgrade from Exchange Server 2019 to Exchange Server SE RTM, as it is considered “low-risk.” For more details, you can refer to this article: Why “in-place upgrade” from Exchange 2019 to Exchange SE is low risk | Microsoft Community Hub 

To clarify the difference between the two types of upgrades, you can refer to below image. 

For additional guidance on upgrading from Exchange Server 2016 to Exchange Server 2019, you can use Microsoft’s official assistant tool: Microsoft Exchange mail management guides 

Furthermore, this thread from another community member provides a real-world example of upgrading from Exchange Server 2016 to Exchange Server 2019 in a hybrid environment, which may offer additional context: Exchange full hybrid replace 2016 server with 2019 | Microsoft Community Hub 

I understand this may not directly resolve every aspect of your situation, as Microsoft’s official documentation often focuses on general scenarios. However, I hope this information provides additional insight and helps guide you toward a solution. 

If you have any updates or further questions, please don’t hesitate to reply to my answer.

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". 

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.   

Thanks, MS releasing of Exchange SE RTM seems very rushed before the EOL for Exchange 2016 and 2019. Give's very little time to migrate.

Also seems a bit odd to suggest to go from EX 2016 to SE is to install 2019 on a new server and then do an inplace upgrade to SE. Why not just install SE straight off, RTM can coexist with 2016.

We have no mailboxes to migrate, but I expect there will be other processes to follow so can't just assume its straight forward!