SET XACT_ABORT ON
BEGIN TRANSACTION;

;WITH
users_source
AS
(
    SELECT users_source.* FROM (VALUES
      ( 'DOMAINOldUser', 'DOMAINNew.User')
    ) users_source ([UserName], [NewUserName])
)
MERGE [ReportServer].[dbo].[Users] AS T
USING users_source AS S
ON T.[UserName] = S.[UserName]
WHEN MATCHED
THEN UPDATE SET
  T.[UserName] = S.[NewUserName]
OUTPUT @@SERVERNAME AS [Server Name], DB_NAME() AS [Database Name], $action, inserted.*, deleted.*;

ROLLBACK TRANSACTION;
--COMMIT TRANSACTION;

GO