we are going to change our email domain let say from firstname.lastname@example.org to email@example.com simultaneously all users email accounts will change. How it can be done seamlessly without disturbing end-users much. we are using hybrid office 365 and all our mailboxes are in office 365 whereas for directory synchronization, we are using AAD Connect tool.
This comes down to your Project planning, and implementing this badly will effect your users...so make sure you plan this carefully.
Things to take in to account: - Adding newname.com as a domain in the O365 tenant <<link>> - using an exchange address policy to add domain as secondary email address <<link>>
- At some point you'll then want to switch from old name to new name to be primary email address.. and can be done with the same address policy.
Have a plan for what's happening with users UPN, is that changing too?, this will need to be communicated to your users in advance and worked through. (some times I've seen this change take up to 24hrs to see the change in O365 as well).