Thanks Canon... I used my 1Dx on my 600F4 with 2x converter, in manual focus at the last aereobatics event I shot. When I ordered the body, I understood I needed to switch to one of my1Ds for auto focus at 1200mm. Now the 1Dx matches the 1Ds in that it uses only the centre spot at F8, so servo tracking means you have to keep the target centred. Now if only my 7D did same.
As for comments about updates, and why this was not included out of the box. I have had TWO firmware updates for the 1Dx since I got it, the 1Ds bodies have each had a number of updates, then there was the major update for my 7D.
Instead of complaining that Canon made a change, we should thank them for continued support and expanding the product's functions. I love new updates to products, especially when they add features, and would prefer they release tested firmware without rushing it out the door.
Then there is of course there is Microsoft and their never ending Windows updates. :-)