All the US megacorps tend send me emails saying "We want to change TOS, here's the new TOS that's be valid from date X, and be informed that you have the right to refuse it" (in which case they'll probably terminate the service, but I'm quite sure that if it's a paid service with some subscription, they would have to refund the remaining portion) - so they can change the TOS, but not without at least some form of agreement, even if it's an implicit one 'by continuing to use the service'.