How to Upgrade the MOSS 2007 Databases from SQL Server 2005 to SQL Server 2008

Hi All

We have a Simple MOSS application using SQL Server 2005 databases [Configuration and Content].It is using Single Farm. We have recently build a new Database Server SQL Server 2008 although the old SQL Server2005 will keep on running. We want to change all the configuration and content Database from SQL Server 2005 to SQL Server 2008[new DB server] for our MOSS 2007 web application.

Can any one provide us the steps involved in migrating the MOSS related database from SQL Server 2005 to
SQL Server 2008 ?

We used the below steps :

1)Took the complete backup of Configuration DataBase and Content Database from SQL Server 2005.
2)Restore the back on the new server i.e. SQL Server 2008.
3)Now from MOSS >> Central Admin >> Operations > Default Database Server 
     Updated the Database server name with the new SQL Server2008 DB instance.

But when we pressed Ok Button it is shooting an error.
Do we need to dettach the DB from the existing MOSS set up , how ?

thanks in advance
Nikhil