What are the implications of changing the SQL Server Service Logon Account? What are the implications of changing the SQL Server Service Logon Account?
My default instance currently uses Network Service.
I want to change this to a domain account.
Are there any implications?
If you are talking about the immediate impact, then the service will need to be restarted in order for that change to take effect.  As per TechNet:


  When you change the service startup account for the Database Engine and SQL Server Agent, the SQL Server service (the Database Engine) must be restarted for the change to take effect. For more information, see Additional Considerations for Changing Service Accounts below. When the service is restarted, all databases associated with that instance of SQL Server will be unavailable until the service successfully restarts. If you have to change the service startup account of SQL Server or SQL Server Agent, make sure that you do so during regularly scheduled maintenance or when the databases can be taken offline without interrupting daily operations.


Using a domain account as the SQL Server service account is considered best practice, so you are making a step in the right direction. 

For further reading, the TechNet reference on Change the Service Startup Account for SQL Server.