Analogical proportions are statements of the form “α is to β as γ is to δ”, noted α:β::γ:δ, and can be understood as “α differs from β as γ differs from δ” and conversely “β differs from α as δ differs from γ”. In this paper, α, β, γ, δ are supposed to be propositional logic formulas, which are appropriate for representing concepts. There exists one approach, developed over the last 15 years, where “α differs from β” is understood in terms of the negation of the material implication α → β. The paper investigates another view where “α differs from β” is interpreted in terms of transformations where some variables become false, some variables become true, and some variables become irrelevant. Both approaches satisfy the three basic postulates of analogical proportions (reflexivity, symmetry, and stability under central permutation), as well as other interesting properties such as transitivity and unicity of δ such that α:β::γ:δ. However, the two approaches depart from each other since they do not validate the same analogical proportions. In particular, when p,q,r are atoms the proportion p:(p∧r)::q:(q∧r) holds in the new approach, while it fails to do so for the other. The new approach exhibits also a good behaviour with respect to integrity constraints. It is advocated that this makes it appropriate for handling analogy between concepts, while the other approach has proved to be fruitful for Boolean features-based representations. The paper provides a thorough analysis of the differences between the two approaches.