Halliday & Matthiessen (1999: 263):
In the semantic construction of experience, 'process' and 'participant' emerge as prototypical categories; and there is a broad agreement among different languages both about the nature of this distinction and about which particular phenomena should be assigned to which category.
But as in any semiotic endeavour there are always some domains of uncertainty: are rain, wind, thunder processes or things? are fear, worry, regret processes or qualities? Examples like these prevent the categories from being too reified and rigid, and provide a kind of gateway of analogy through which a phenomenon can drift or be propelled from one category to another.
In transcategorisation some other semantic feature triggers the propulsion; e.g. dark + make/become = darken, flake + like/composed of = flaky.
In metaphor, however, the phenomenon is reconstrued as another category; what is being exploited is the potential that arises — but only after the categories have first been construed as distinct; not otherwise — of treating every phenomenon in more ways than one. In this process the original interpretation is not supplanted; it is combined with the new one into a more complex whole.