1-Copy the databases from the old SQL server to the new one.
2-Then, in the config database on the new SQL server, open the Servers
table in the config database.
3-Change the Name and Address fields that have the old SQL server name
to the new SQL server name.
4-Open the SharePoint Central Administration page and click on “Set
configuration database server” under the Server Configuration section.
5-Change old SQL server name to new SQL server name. Click OK.
6-Again on the SharePoint Central Administration page, click on
“Configure virtual server settings”, and then click on a website you
want to change.
7-Click on “Manage content databases” under Virtual Server Management.
8-Click “Add a content database” and then ensure “Specify database
server settings” is selected.
9-Enter the SQL server name and database name. Enter 100 for each of
the following boxes and click OK. (Notice the content database that
was already there will have a 0 under the “Current Number of Sites”
column. This is because SharePoint realizes you are trying to
substitute. You can then remove the old database from the WSS list.)
Repeat steps 6-9 for any additional sites.
I know there are several steps, but it is truly the easy way. It goes
much smoother than any other way (in my experience).
Ensure the account used for the Application Pool has rights to the
databases (both config and any content db).
Note: Editing SharePoint databases can make your configuration ‘unsupported’ by Microsoft