Exchange Hybrid Migrations: More Than Just a Pretty Face

PJR_CDF , you would normally need to install Exchange 2016 and point the MRSProxy namespace (EWS) to the Exchange 2016 server version. Exchange 2010 wouldn't be able to proxy to Ex2016 if you were for example to move an Ex2016 mailbox to Exchange Online. Also HCW would detect the Exchange 2016 as optimal Exchange server.  Also for Free/Busy coexistence,  the bridgehead server requires Exchange 2016 as OAuth servers.  But we also don't have a clear statement on the coexistence Exchange 2016 /2013/2010 for Multi-tenant hybrid. My recommendation is to wait a bit until we clarify these things officially with our Engineering Team.

As a side note, I was able to test migration successfully in my Exchange 2010 lab organization with the second tenant in a Minimal Classic Hybrid configuration (first tenant having Full Classic Hybrid).