With an SQL Alias, you create a pointer to the new SQL Server. That's why your Central Admin Page still thinks it's using MyOldDBServer. The alias lives in the Operation System and is not part of SharePoint. The same goes for any other Non-SharePoint-application on the same server. The alias takes control of where the Database Server is - regardless of what the application says. Thats't the beauty of it.
When you add an SQL Alias to your server hosting Central Admin, you need to move ALL the databases to the new storage (provided that they are using the same database server)
The SQL Alias has nothing to do with the SQL Sever. It is a server feature where you point all your application to a different SQL Server. Not just SharePoint. What yoo do is that you tell the Server OS that MyOldDBServer from now on is located at MyNewDBServer. You don't have to change anything within applications; SharePoint or any other. SharePoint and other apps, still belive that they are communicating with MyOldDBServer.
Try the following CMD command on your computer:
- Run CLICONFG (on windows 7)
- Start Screen type CLICONFG (on Windows 8)
And you see this dialogue:
And you add the alias on the Alias tab:
Well almost, backup the databases and then you have to shut down SharePoint through the action of disable services and stop the ISS before adding your SQL Alias. But that part you already know. If not - follow Todds blgg Moving SharePoint to a different SQL server.