Originally Posted by Leingod

SO that command does run. But it doesn't make any notable changes to the settings of the cloud mailbox. Makes no sense to me. I do have a solution though and should probably make a post about it somewhere online because I very well might be the first person in the world to have done this.

What i did was first disable the on-prem mailbox. I also made sure to ignore any legalholds and set PS to permanently disable just the mailbox.

I then enabled the remote mailbox as well as the remoteroutingaddress for the cloud account.

I then ran get-mailbox commands from both on-prem and Online to do a stare and compare and see what difference were between a working users account and the new remote mailbox account.

I had to make a single change to our default MDM policy for the new remote mailbox.

I then exported a PST of the on-prem disabled mailbox.

Looking in Exchange, I could confirm the mailbox is now pointing towards the cloud and I can confirm from EAC that the mailbox is migrated properly.

I then had to re-add all known SMTP aliases to the Exchange profile and perform a delta sync within AD Connect to get the names to begin appearing for the cloud mailbox.

This allowed for proper mailflow again for the users account.