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