Montreal-based Astral Media Inc. and Corus Entertainment Inc. of Toronto announced yesterday they plan to exchange a number of radio assets in the province of Quebec. Under the terms of the agreement, Astral's seven AM stations, including the Radiomedia network, and CFOM-FM in Quebec City are to become part of Corus's radio division. In exchange, five Corus-owned FM stations in Rimouski, Amqui, St. Jean-sur-Richelieu and Drummondville will become part of Astral Media's radio division.