Thanks to Rob Hyndman for pointing me to the following article on Findlaw.
A recent FTC consent order may have significant repurcussions for those in the United States who may want to change their online privacy policies to allow them to use information in new ways.
Modern Practice - Privacy Policies: Beware of Changes:
"By Justine Young Gotshall
© 2004 Wildman, Harrold, Allen & Dixon LLP"
Canadians will want to take a look at Kanitz v. Rogers Cable Inc. (2002), 58 O.R. (3d) 299 (S.C.J.), which gives more latitude when changing online contracts, as long as you have given notice that you may do so from time to time.