Halliday & Matthiessen (1999: 259-60):
Rank shift is not inherently metaphorical. There is a parallel here between rank shift and class shift. In origin, both these could be described as metaphorical semogenic processes: a verb or adjective is metamorphosed into a noun (a shift of class, e.g. strong : strength, lose : loss), a clause is metamorphosed into a group (a shift of rank, e.g. they went bankrupt : their bankruptcy). But as a synchronic relation neither of these necessarily involves metaphor; there may be no systematic alternation such as there is between a metaphoric and a congruent form. We have already discussed non-metaphorical forms of class shift, under the heading of transcategorisation. […] Class shift becomes metaphorical when the “shifted” term creates a semantic junction with the original.