SciFi Wire has posted an article that features an interview
with Scott Benza and Scott Ferrar from Industrial Light & Magic (ILM). Within the interview, the two supervisors from the first two Transformers films talk about the third film. Scott Benza confirms that Optimus Prime and Bumblebee's robot modes will receive subtle changes. They believe that Bumblebee has matured through the first two films and will receive an update to make his physique look more mature, while Optimus Prime's update will make him look more heroic.
Elsewhere in the interview, Scott Ferrar mentions that they have an outline of the movie in hand and that the script is mostly done. The ILM art department is currently working on turning 2-D work into the 3-D characters.