Simple: more CGI alt and robot modes. Quite possibly, we could be getting more military Decepticons this time around, so it wouldn't surprise me if they had CGI alt modes when in cities. Keep in mind, most Decepticons have very "alternate" alternate modes, if you know what I mean. They're not just cars and trucks. There's a big chance that if the Decepticons DO have physical alt modes during the filming of TF3, we won't get to see most of them. That's the way it was with most of the Decepticons for TF1, and the Constructicons with ROTF. That's probably the way it will be this time.