Describing scientific change involves displaying scientific theories and their content. This is done by way of (rational) reconstructions, which usually neglect the variety of versions in which a scientific theory may appear. Although they possess a unity allowing them to bear the same name for decades, or even centuries, theories are diachronic, evolving entities. How should their evolution, i.e., into-theoretical change, be described? I claim that this description is best done in terms of a series of conceptual undertakings on the part of scientists aiming at examining the meaning of theoretical principles and concepts and broadening their domain. I present the implications of this hypothesis, which follows Robert Brandom's views on the philosophy of language, as well as the conception of scientific change in general associated with it.
