Demystifying Exchange Online Provisioning: Architecture, Exchange Object Types, and Attributes

Satyajit321 Thanks for your comment.

The ExchangeGuid is not written back to Exchange on-premises. Suppose you have created a remote mailbox in Exchange on-premises, this will create the remote mailbox object in Exchange on-premises with an ExchangeGuid without value, which will allow Exchange online to assign one that is not in use. If at some point you would like to offboard that mailbox (migrate it to Exchange on-premises from Exchange online), then you would need to set it manually in Exchange on-premises before you inject the move by running Set-RemoteMailbox <id> -ExchangeGuid <ValueFromExchangeOnline>.

Set-ADUser can't be used for EXO DS but on your on-premises environment, however, as described on the article we don't support doing that.