SQL Server, convert a named instance to default instance?

As far as I know, no. One reason is the folder structure on the hard drive; they will have a name like MSSQL10.[instancename]

This is why a lot of companies store their applications' connection strings at the machine level instead of the application level.

Just take the connection string out of the source code entirely. Then have everyone put their connection string in their machine.config.

This has the added benefit of avoiding unnecessary app-specific environment logic, i.e. when you copy your application to the staging server, the staging server already "knows" what database to use.

I also wanted to convert a named instance to default - my reason was to access it with just the machine name from various applications.

If you want to access a named instance from any connection string without using the instance name, and using only the server name and/or IP address, then you can do the following:

• Open SQL Server Configuration Manager
• Click SQL Server Network Configuration
• Click Protocols for INSTANCENAME you want to make available (i.e. SQLExpress)
• Right-click TCP/IP and click Enabled
• Right-click TCP/IP and go to Properties
  • Go to the IP Addresses tab
  • Scroll down to the IPAll section
  • Clear the field TCP Dynamic Ports (i.e. empty/blank)
  • Set TCP Port to 1433
  • Click Ok
• Go to SQL Server Services
• Right-click your SQL Server (INSTANCENAME) and click Restart

This will make the named instance listen on the default port. Note : You can have only one instance configured like this - no two instances can have same port on the IP All section unless the instance is a failover cluster.