Change Office 365 host from BT to Microsoft

Hello John,  

Thank you for your quick response. As there are 6 active users, and the Custom Domain is being used in BT, you can still add the same custom domain in O365. Both companies have different architecture. Please make sure you download everything (Mails, Calendar Items, Meeting Requests) in one single .pst for individual users on your local pc before making any changes to BT user Accounts. 

Create a New Tenant in O365 by Signing Up with your Custom Domain and create 6 users with same Display Name, SMTP (primary Email address) etc., fields manually. Create a different password for the time being so that you would be sure while configuring Outlook that its taking the settings of O365 and not BT as those accounts are also active. Once you disable BT Accounts, you can re-change old passwords of those users. Its recommended that you don't make any changes like disabling User Account, Licenses from BT etc until you are sure that all the information required is replicated to O365 Mailboxes. 

Assign the license to all those users to create the mailbox in O365.

Configure Outlook with new profiles on local pc. You can use SARA Tool for configuring it. I highly recommend you to do it. I am providing you the link below: 

https://diagnostics.outlook.com/#/

Download it and run it on the pc to configure Outlook with new profile for O365 users.

I hope performing all the above tasks may be time consuming, however you will be able to complete the migration easily.  Still in case if you need any help, I am always here to help you out. You may enjoy a cup of coffee in between while downloading of .pst, importing of .pst after configure new profile in Outlook for O365 users and then those changes getting replicated to cloud in same mailbox happens.

Thanks & Regards,

Ajey Kumar Gupta

Microsoft Engineer.