Exchange 2010 to 2016 Migration Outlook Client issue

Hello, I am at the start of migrating users from 2010 to 2016 all in house servers nothing on O365 Etc. I have done two single mailbox test migrations with some strange results.

The clients use Outlook O365 Apps that have been connecting to the 2010 server.
So the issue I am seeing is, the mailbox migration is successful, users gets the pop up "Admin made a change please close outlook and reopen". But the actual job is not complete just the mailbox migration within. So they close and open and they get nothing, can't connect. 
OWA works fine, mail is getting and receiving but the outlook client is not happy.

Servers:
2010 Server Exchange on 2008 R2 all patched up
2016 Server Exchange on 2016 all patched up

I did some investigating and I found an error on the PC, Application events. outlook. showing it changing the DSN.

Problem is it changing the DSN to the same server name. 

It should be 

Old DSN=Exchange  New DSN=Exchange16
but in the logs it's
OldDSN=Exchange NewDSN=Exchange

But then about 24 hours later the users comes in and all is working.

I checked all aspects of the client config and server and DNS. All pointing to new server as well internal external url's are all good. Everything works again after the 24 hours and I can't seem to find where the DSN is getting updated to the client to allow the connection.

Any ideas would be appreciated.