Well maybe I am really thick today as I could not link this properly either. This reply (below) is useful and I rang apple UK who said that my films would stream as 4k on an appropraite device, otherwise e.g. offline it would be HD. Best thing is to try it I suppose but I am going to wait until after next week's conference. Thanks everyone for contributions.
Sep 30, 2017 4:03 PM in response to fourenvy
Just had an interesting conversation with an apple chat advisor who informed me that not all previous HD movie purchases will automatically be available to you for free when its made available in 4K.
I have previously purchased Labyrinth in HD.
The movie is now on the 4K store.
I am asked to purchase this move instead of being able to stream it in 4K.
Apparently this is because the content provider has modified (whatever that means) the film so I have to purchase it in 4k as well if I want to watch it.
Its worth stating; I do have a number of previous HD purchases that are all working free of charge in 4K without issue. Man of Steel, Prometheus, Batman vs Superman and Despicable Me are all streaming in 4K and not additional payment has been made.
I have added some of the transcript for info.... Very frustrating!
Mark Humphrey
I have just noticed that the movie Labyrinth is now available in 4k ... i have this movie in HD but when i attempt to watch it in 4k i have to purchase it???
Sharan
Yes, currently as of now, you will not be able to switch the movie, you will need to pay for it.
Sharan
I have carefully reviewed UK iTunes Store and found that those movies have been modified on UK iTunes Store that is the reason you are unable to watch them in 4K.
Sharan
I see that you have purchased the movie “labyrinth” in 7.99 GBP and now the price of the movie is changed to 6.99 GBP.
Sharan
Since the content is modified by the content provider that’s the reason you are unable to play in 4K.