I was curious about when this change took place also. Only because I remember having a cheeseburger at the ASMu food court and the bun seemed fine to me. Maybe not the most magical bun but, I would remember if it was inedible...
To the posters asking where these buns were, I believe it's been said here and in several other threads that these buns only seem to be at the Counter Service restaurants.
I was just at Sci-Fi last fall and remember having a terrific burger! I will miss it this year but, we're doing Prime Time this trip as it's only DBF's second trip so, must try new restaurants for him.