Hi James,
Before moving on, I’d like to confirm the following things:
1. Do you have you own local AD and the DirSync works well at the same time?
2. Where is the mail service hosted? Are all the mailbox on-premise or some on-premise and some in Exchange Online or any other situation?
3. Do you want to change an Office 365 tenant ? Or do you just want to change your current custom domain or onmicrosoft.com domain to another one?
4. Do you want to remove all the current on-premise environment?
In addition, I’d like to confirm if you get a Service Request number when you raised a call.
If so, please send me the service request number in private message, so that we can check the process of this issue. I have sent you a private message to collect the information.
To read private messages, you can do the steps below:
.Go to the Your details section on the right side of the community site.
.Click Private messages.
.Click the subject title of the response to read the message.
.You can reply by using the form in this display.
.Click Submit Reply.
Thanks,
Vivian Wang