This is actually fairly common on some routes. They tend to switch equipment allot to certain destinations and don't want to sell Elite/Deluxe on the chance they may decide to go with an A330 between now and the time the flight goes out. They only sell 2 classes. If there is an equipment change you loose your seat anyway so check back periodically but I don't think you are any more likely to loose an Elite seat than any other seat.

And being gold and having lost my seat assignments a few times lately. I don't think status has anything to do with it. They let you pick the seat and it is yours. If they change equipment, you loose your seat just the same as anyone else and they don't seem to always handle that well.

In fact the most recent example where I lost my seat assignments involves that sort of scenario. TPE-MNL originally scheduled on an A330. I had exit row seats reserved. They changed the A330 to a combi and I found I had lost ALL my seat assignments on ALL of my flights. Simply went to unassigned and if I hadn't noticed a head of time I suppose I would have found myself getting what ever was leftover at check-in. I called and got my seats back including a Deluxe seat in the nose of the Combi on my economy ticket TPE-MNL. But the Deluxe seats were available to anyone.