SET XACT_ABORT ON

BEGIN TRANSACTION
BEGIN TRY


    SELECT Row_Number() OVER (ORDER BY TableOneId DESC) -1 AS Offset, TableOneId, A, OtherFields
      INTO #TableOneUpdates
      FROM TableOne
     WHERE A IS NULL
  ORDER BY TableOneId

    DECLARE @TopId INT

    INSERT INTO Table2
    SELECT Values FROM #TableOneUpdates
    ORDER BY TableOneId

    SELECT @TopId = SCOPE_IDENTITY()

    UPDATE #TableOneUpdates SET A = @TopId - Offset

    UPDATE TableOne
       SET A = T.A
      FROM TableOne
INNER JOIN #TableOneUpdates T ON T.TableOneId  = TableOne.TableOneId

COMMIT TRANSACTION
END TRY
BEGIN CATCH
ROLLBACK TRANSACTION
END CATCH