I see the possibility of a mod/add-on author stating that they'd need "X" amount of dollars to cover their 'costs' (based on time invested on development, etc) and then converting the mod/add-on to an unpaid/free resource.
Although I don't think it should be an easy process as there should be some sort of disclaimer/disclosure agreed to with strict guidelines as to when they can or can't change it. For example, if they choose to move it from paid to free they would have to agree to a "promotion period" of "X" number of days when it can be 'free' (or at a discounted rate). This 'time period' should have a counter and should be displayed as well for consumers to have the information needed and it helps instill a sense of urgency on the purchase/download as the promotion would 'end soon'.