How to update the SSRS Users table after a username change?

My user name has changed from jdoe to john.doe (for example). The reference in the [dbo].[Users] table shows my old user name, jdoe.

However, with my new user name, john.doe, the subscriptions fail to run and throw the error:

Failure sending mail: The user or group name 'ABCDE\jdoe' is not recognized.Mail will not be resent.

The [dbo].[Subscriptions].[OwnerID] value references the [dbo].[Users].[UserID] for jdoe.

Can I simply change the [dbo].[Users].[UserName] value to my new username, leaving the [dbo].[Users].[UserID] and [dbo].[Users].[Sid] columns as is?

I would use a MERGE statement for this. Check the transaction on a rollback to test with first. You'll also need UPDATE permission on the dbo.Users table.

SET XACT_ABORT ON
BEGIN TRANSACTION;

;WITH 
users_list
AS 
(
    SELECT users_list.* FROM (VALUES
          ('DOMAIN\OldUser1', 'DOMAIN\New.User1')
        , ('DOMAIN\OldUser2', 'DOMAIN\New.User2')
    ) users_list ([UserName], [NewUserName]) 
)
, 
users_source -- don't add users that already exist to prevent duplicates
AS 
(
    SELECT 
          [UserName]
        , [NewUserName]
    FROM
        users_list 
    WHERE 
        1=1
        AND [NewUserName] NOT IN(SELECT [UserName] FROM [ReportServer].[dbo].[Users])
)
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