Impacts of changing default domain in Office 365 tenancy?

Thanks for the add Dean_Gross - yes, we've looked into this extensively for six+ months.

I've been focusing on the impacts to SharePoint/OneDrive, while others in my IT department are focusing on AD/Exchange/Skype.

My first suggestion: contact Microsoft support.  Microsoft has a process that they are currently refining to allow a tenant to change names.  Most organizations will find changing the tenant name to be disruptive, especially large and change sensitive cultures like my company - so don't underestimate the effort.

In fact, changing the SharePoint/OneDrive URLs was too disruptive for us, so we're going to wait until next year to see if Microsoft has a better process.

In the meantime, we've added our new domain as our primary SMTP address, but left the UPN and SIP set to the old address.  This can be confusing to users because almost all login prompts ask them to enter their "email address" when the prompt is actually for their UPN.  We haven't had but a few calls about this since we added the email addresses a few months ago.

We're currently testing the UPN change, which will change the URL of the user's OneDrive site (among other things).  Changing that address will break things like:

1. OneNote notebooks that are open in the OneNote desktop app - users will need to notice that their notebooks are no longer syncing.
2. Office recently used documents list - the links to recently used documents that are stored in OneDrive will break, because the links will continue to point to the old address.  Our testing shows that this isn't 100% of the time, but we haven't tracked down why some users are reporting that their links continue to work.
3. Office Pro Plus login issues - I don't have the details here, but some of our people have had trouble with the Office apps not being able to login (if I remember correctly)
4. Bookmarks/favorites/etc. - any links that someone saves, including the sharing emails they receive, will no longer work.  The permissions are left untouched, but the link in the original sharing email will no longer work.

One of the good things is the OneDrive desktop app - it will "heal" itself after a logout/reboot and doesn't require the user to do anything.

We've decided to proceed with the UPN change because we need to minimize the amount of time that our UPNs don't match what our users consider to be their email addresses.  Plus, changing the OneDrive sites won't be as bad as SharePoint because most of our sharing/active documents are in SharePoint.  Lastly - we can implement the UPN change over time, giving our help desk more time to deal with issues.  When we do the full name change, including changing the base address for SharePoint Online and OneDrive, we'll have to do that in one step and incur a high call volume for help desk.