Side By Side Upgrade and Migration from 2008 to 2016 Side By Side Upgrade and Migration from 2008 to 2016

Old Server Name Test1

New Server Name Test1

Default Instance Test 1

My question is when the new server be named as the old one 'Test1" and it will try to register the dns while the old server is online wont be there a problem as the name is already being registered. In that case what will be the workaround since it is a side by side upgrade and migration I need to use the same server name and need to use  both the servers in order to upgrade and mirgate and then turn off the old server after the switch over
I've had to do this and just had a record available for the old server.  Renamed the DNS entries and did a ipconfig /flushdns on the servers themselves since they were replicating back in case of a failback.  We set the DNS to a very low TTL so clients also got updated.  Powershell made all this very easy and fast.

If your clients are easily identified and you have access, you can do a flushdns yourself on them as well.  I recorded and documented all connections going to the database prior to the upgrade so I knew all the clients to look out for.  Then finally I set the old database to block all users except the sole one I was using for replication on the old machine and ran traces to see if anyone was attempting to connect to the old instance.  None did but if they were, I would inform them to update DNS or check their connection string.