Changing the LoginName of a user

I'm trying to automate the creation of a databse and a User/Login but i'm running into problem with a conflict between a new user to which i'm trying to assign a WindowsLogin which is already linked to the dbo. (SQL Server 2005)

Is there a way to reassign the dbo's LoginName to some othe user

Here the code

IF NOT EXISTS (SELECT * FROM sys.database_principals WHERE name = N'ABC')
BEGIN
declare @.cmd nvarchar(max)
set @.cmd = 'CREATE USER [ABC] FOR LOGIN ' + @.WindowsLogin + ' WITH DEFAULT_SCHEMA=[Shared]'
exec(@.cmd)
END

Which give me the error

Msg 15063, Level 16, State 1, Server SR-DEV-GOI1, Line 1
The login already has an account under a different user name.

The script is running under the @.WindowsLogin in question !

Thanks All

Gilbert

** Reply to myself **

I have found the way to change the loginName of the dbo user with sp_changedbowner but then it restrict the access to the database. We're gonna use an other Login to create the process (which makes more sense)
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The system SP sp_changedbowner uses the ALTER AUTHORIZATION DDL among other things. You can use the DDL instead to perform more granular operations including changing database ownership. See BOL for more details.