CRM Upgrade 3.0 --> 4.0 Problem - GrantCrmDatabaseAccessAction failed

We received the following error:
Action Microsoft.Crm.Setup.Server.GrantCrmDatabaseAccessAction failed. The login already has an account under a different user name.

This happens while the wizzard is updating the system and after that, the repair-function does also not help to resolve this problem.

I found a hint here.

Solution:
Change the db-owners of the crm databases (MSCRM and METABASE).

Code-Snipped:
EXEC .dbo.sp_changedbowner @loginame = 'sa', @map = 'true'
EXEC .dbo.sp_changedbowner @loginame = 'sa', @map = 'true'