I'm a travel agent at the moment and do a lot of business with Emirates. Emirates have an un-written company policy to look after frequent passengers particularly well - whether they hold a Skywards card or not. I would assume that when your travel gals book you on to EK, a note is sent to their Emirates rep asking for the upgrade. AS you book club now and again, and use the airline frequently, it's in their own interest to sit you in an empty club seat.
Keeps you going back, right?!!?
Usually, a Y-J upgrade wouldn't be made available but now and again, this does happen.
What confuses me though is that it's the crew members who move you. If the ground crew upgrades you, you will be issued a new boarding pass with the new seat number on it, as you pass through the gate. It sounds to me that perhaps the decision is left up to the crew, in this instance.
If you book a Business Class ticket though, make sure you tell your travel gals to speak to their rep about possibly upgrading to FIRST, as this is quite a regular thing with EK. Book Club, travel First.
If you can get this on their new A340-500, you'll be laughing!!!!