If for any reason you choose to install MOSS 2007 with a local SQL Express database and later find that you need to upgrade it to MS SQL 2000/2005 Server, you have two choices:
Perform an in-place upgrade of SQL Express to a local MS SQL server. This is easy.
Move from SQL Express to a separate MS SQL server. This is a complicated process.
There is a quick guide for an in-place upgrade to SQL Server 2005. Be sure to follow Microsoft's detailed instructions for upgrading more complex SQL Server scenarios. The guide can be found at: http://msdn2.microsoft.com/en-us/library/ms143516.aspx.
This section describes the first option only: an in-place upgrade. If you want to move from a local database to a remote database server, you must check for a migration guide in Microsoft's Knowledge Base.
The most important thing to do before starting the upgrade process is to back up your current MOSS database. You are about to perform a very sensitive operation, and if something goes wrong, you must be sure you can go back to the previous version. The previous section described how to back up your database.
During this upgrade process, no one can use the MOSS server. Make sure to pick a time when no one is using the server. Since the SQL Express version that comes with MOSS only allows up to 4 GB of data, this upgrade process should be fairly quick, depending on your server hardware, but plan for at ...