Take a macro model of an economy with two different goods being produced; call them Carrots and Grapes (there can be many different varieties of carrots, and many different varieties of grapes, if you like). Suppose the demand for grapes falls, because of a change in preferences. Will there be an equal and offsetting increase in the demand for carrots? Well, that depends. It depends on what the central bank is targeting.
Suppose the central bank targets 2% Carrot Price Inflation. And suppose the central bank gets it exactly right, and responds perfectly to the preference shock, so the Carrot Price Inflation rate stays at exactly 2%.
What happens to real GDP?