How does this work in practice though ? If a flight was full in Y, but had seats in J, and you used your EK Gold status to grab a Y seat - presumably someone (perhaps you) would be op-up'ed to J to make room.

In the same example, if both Y and J were full, I presume they chuck someone off the flight to make room for an EK Gold member using the "Guaranteed Seat" privilege ?

All classes in the flight need to be full to get this benefit as per the T&C. So your example of Y being full and J having space does not qualify..
But this is the only thing clear from the T&C's.. As evident in this thread there are many many unknowns or reasons given from time to time and CS center to center.

It seems quite a few posters had problems with semi-informed agents.

I always call the German call center number, the agents are very capable and always know exactly what to do.

Good to know but for many of us this would involve considerable cost considering that these kinda calls dont end quick from my experience..

Btw, did u get clear answers and rules for this benefit from them ? If yes, kindly share here for everyone's benefit since I have got conflicting answers and I see so have others..

Why don't you write to Skywards asking for these exact terms and conditions and highlight your experience with the two calls and your utter frustration and disappointment. See what they say.

A bit OT but I find it strange how the airline industry is one of the few where companies can get away with dissatisfied customers. I had a concern (not even a complaint, just a concern) about a product I'd purchased recently and the company offered to replace it, within about 3 days of my writing in to them. Premium air travel is a premium product and one expects a similar level of customer service. Ok but the margins on premium air travel are much smaller than on other premium products.

Will surely be doing this..
Will update once I get any kinda reply ..

Does Ek charge the usual change fee if they change a flight because pax has invoked Gold Guarantee seat and they have changed the flight ?

In my case they did.

I had asked for a gold guarantee for a return leg which was booked on flex ( the ongoing was saver)..
Initially, they said that its only possible if the full itinerary is on flex, but I said thts not mentioned in the T&C's.. then other excuses..
Then suddenly I got an email asking me to do olci for the other flight I had asked for without any information. I went to my bookings and saw that both the flights were being shown in my bookings and it was not allowing me to do olci..
So I called EK and they said they have confirmed my seat on my requested flights and I needed to pay the change fee .. I did that and was able to do olci then..
Things to note here for future benefit of others :
1. One leg was in Saver and other in Flex.
2. The return leg ( on flex) consisted of one flt ( XXX-dxb) which was fully booked and the next ( dxb-xxx) which had space available. ( This experience is different than what B747 experienced and goes to show how many open questions are there in this benefit)..

Morale of the atory -- persistence pays on some occasions !!

Did you change both flights?

Moral of the story is that EK loses business because they can't get their act together. The concept of calling back until you get the answer you want is ridiculous. It isn't a benefit if you have to jump through hoops like a show dog in order to avail of them.

Yes, The first flight was completely full in all classes and the 2nd one quite empty..

+1
My morale of the story rose because of what u wrote