Can I ask why you prefer EXCEPT over NOT EXISTS?

A stored procedure can have more than one statement in it. If you want to break it down into two simple actions, you can.
-- Update existing rows:

I like Jacob Wilkins's solution. And I'd go a step...

Humbled.

Well refer to my last post and see if you can sort out the ORDER BY on your own. I'm sure you can. 🙂

Maybe you need to cast the whole "date + time" as a proper DateTime and sort by that? 

WITH CTE AS
(
SELECT
  ...

I'm really not sure how you want it sorted, would you like to explain it to us?

If you need help ordering the results, try the ORDER BY statement. 

To accomplish the other part, you will probably need to use a CASE statement in the second...

This?
SELECT
    CAST(WorkDate AS VARCHAR(25)) + ' ' + CAST(Starttime AS VARCHAR(25)) AS WorkDateTime, 
     EmpID, 
     EmployeeName, 
     Code...

Perfect case for the sexy COALESCE function:

SELECT
   COALESCE([CYsales].[cust_id], [LYsales].[cust_id]) AS [cust_ID]
, [CYsales].[sales]
, [LYsales].[sales]
FROM [CYsales]
FULL OUTER JOIN [LYsales] ON...

Jeff is awesome.

I love you Jeff Moden.

Gotcha, thanks Luis and Jeff as usual for sharing!

Luis-
I understand what a Cross Join is, and I understand what your solution is doing, but I'm a bit unclear on what you mean by "unintended cross joins". Can...

I'd say that, if you ALWAYS want to cascade an update/delete, then enable it. If there will regularly be exceptions, as it sounds is the case in your situation, then...