Many works have studied preferences in Dung-style argumentation. Preferences over arguments may be derived, e.g., from their relative specificity, relative strength or from values promoted by the arguments. An underexposed aspect in these models is change of preferences. We present a dynamic model of preferences in argumentation, centering on what we call property-based AFs. It is based on Dietrich and List's model of property-based preference and it provides an account of how and why preferences in argumentation may change. The idea is that preferences over arguments are derived from preferences over properties of arguments, and change as the result of moving to different motivational states. We also provide a dialogical proof theory that establishes whether there exists some motivational state in which an argument is accepted.