It seems that the evidence is slowly coming in now about some of the upcoming changes for 9th edition Fantasy. Round bases have been seen now in the photos from the Black Library sample images for epub White Dwarf 50 (correction). Just check out the bases behind Thanquol.... They're Round.
More than likely this was a mistake by a photographer that did not know his models well. Screaming Bells and Plague Furnaces are generally on square bases as you can see in the following images.
Definitely a sign of what is to come. It seems that the models in the studio are already being moved over to the new bases.