Transformation
Transformation is the process by which a Transformer alters his form between modes. It should not be confused with reformatting a type of full-body upgrade.
Maintaining a transformed state requires a constant output of energy. The 'base' state for a Transformer seems to vary- sometimes robot, sometimes vehicle or skirmish mode. Most Beast Warrriors have their beast mode as their base state.
Transformation is a mentally rigorous task, requiring the transformer to focus on reconfiguring all the elements of his body in a way he or she isn't used to thinking about. In the Beast Era, this difficult task was choreographed by an on board computer that reconfigured the transformers body for them. Post-reformatting, the new technorganic forms granted to Cybertron's inhabitants lacked on board computers, requiring them to re-learn the mental disciplines involved.
Robots with multiple transformations (tripple changers, et al.) are a special case. Though there seems to be some aspect of natural aptitude at play in one's ability to master multiple forms- the balance of the matter seems to be one of training. Most six-changers are said to have mastered their plethora of transformations through rigorous study.
Try turning on your back and walking spider-style like that kid in The Poltergeist. Not easy, is it?
Some groups of Transformers such as the Go-Bots use a process similar to reformatting to more radically alter their forms while transforming. This ability comes from unique metals in the Go-Bots' bodies, and does not apply to most Transformers.
Fiction
Marvel Comics
Unicron was the first Transformer, fashioning his metal prison into a robotic semblance of his original form. Primus, watching froma distance was all like- "Oh snap, robots that turn into things? why didn't I think of that?"
