From NavCanada:

NAV CANADA, the country’s provider of civil air navigation services, recently completed an aeronautical study of the airspace in the London, Ontario area. The study recommended a change to the airspace classification at the London Airport.

A Control Zone classification change from Class D to Class C transponder mode C required) will enable more effective and efficient provision of air traffic control (ATC) services at the London Airport.

This change will take effect March 12, 2009 at 09:01 Coordinated Time Universal Time (UTC). The appropriate aeronautical publications will be amended.

Official announcement is in this “PDF”:http://www.navcanada.ca/ContentDefinitionFiles/Newsroom/ServiceProjectAnnouncements/2009/an0114d_en.pdf.