sp_change_users_login

sp_change_users_login @Action='update_one', @UserNamePattern='itai', @LoginName='itai'

GO

Maps an existing database user to a SQL Server login.

@Action can be:

• Auto_Fix - Links a user entry in the sys.database_principals system catalog view in the current database to a SQL Server login of the same name.
• Report - Lists the users and corresponding security identifiers (SID) in the current database that are not linked to any login.
• Update_One - Links the specified user in the current database to an existing SQL Server login.

http://msdn.microsoft.com/en-us/library/ms174378(v=sql.90).aspx