Problème avec alter puis mise à jour dans try catch avec tran en utilisant Transact-SQL

Même si j'écris ma propre réponse - tout le crédit revient à @Mikael Eriksson, qui a suggéré que je devais séparer différents lots avec un GO - afin que le code qui modifie la table n'entre pas en conflit avec le code qui utilise la table modifiée. Merci Mikael !

USE PUK;
GO
BEGIN TRANSACTION;
BEGIN TRY

   -- - Modify RETRIEVAL_STAT
   alter table dbo.RETRIEVAL_STAT add
      SOURCE nvarchar(10) NULL,
      ACCOUNTNUMBER nvarchar(50) NULL,
      PUK nvarchar(20) NULL;

END TRY
BEGIN CATCH
    SELECT
        ERROR_NUMBER() AS ErrorNumber
        ,ERROR_SEVERITY() AS ErrorSeverity
        ,ERROR_STATE() AS ErrorState
        ,ERROR_PROCEDURE() AS ErrorProcedure
        ,ERROR_LINE() AS ErrorLine
        ,ERROR_MESSAGE() AS ErrorMessage;
    IF @@TRANCOUNT > 0
        ROLLBACK TRANSACTION;
END CATCH;
IF @@TRANCOUNT > 0
    COMMIT TRANSACTION;
GO


USE PUK;
GO
BEGIN TRANSACTION;
BEGIN TRY

    -- transform logic.
   UPDATE      dbo.RETRIEVAL_STAT
   SET         SOURCE = 'ABC',
               ACCOUNTNUMBER = ABC.ACCOUNTNUMBER,
               PUK = ABC.PUK
   FROM        RETRIEVAL_STAT RS
   INNER JOIN  ABC
   ON          RS.SERVICE_NUMBER = ABC.SERVICENUMBER;

   UPDATE      dbo.RETRIEVAL_STAT
   SET         SOURCE = 'DEF',
               ACCOUNTNUMBER = DEF.BILLINGACCOUNTNUMBER ,
               PUK = DEF.PUK
   FROM        RETRIEVAL_STAT RS
   INNER JOIN  DEF
   ON          RS.SERVICE_NUMBER = DEF.SERVICENUMBER;

   UPDATE      dbo.RETRIEVAL_STAT
   SET         SOURCE = 'No Match'
   WHERE       SOURCE IS NULL;

    -- Fix other columns that should be not nullable.
   alter table dbo.RETRIEVAL_STAT
      alter column SERVICE_NUMBER nvarchar (50) NOT NULL;
   alter table dbo.DEF
      alter column PUK nvarchar (20) NOT NULL;


END TRY
BEGIN CATCH
    SELECT
        ERROR_NUMBER() AS ErrorNumber
        ,ERROR_SEVERITY() AS ErrorSeverity
        ,ERROR_STATE() AS ErrorState
        ,ERROR_PROCEDURE() AS ErrorProcedure
        ,ERROR_LINE() AS ErrorLine
        ,ERROR_MESSAGE() AS ErrorMessage;
    IF @@TRANCOUNT > 0
        ROLLBACK TRANSACTION;
END CATCH;
IF @@TRANCOUNT > 0
    COMMIT TRANSACTION;
GO