I use this; Run it against the reportserver db.

DECLARE @OldUserID uniqueidentifier

DECLARE @NewUserID uniqueidentifier

SELECT @OldUserID = UserID FROM dbo.Users WHERE UserName = '<NewUser/domain>'

SELECT @NewUserID = UserID FROM dbo.Users WHERE UserName = '<OldUser/domain>'

UPDATE dbo.Subscriptions SET OwnerID = @NewUserID WHERE OwnerID = @OldUserID and laststatus like 'Failure%'

** we also created a ReportSub/domain user account and we use that account in the NewUser spot mentioned above.