I see it the opposite way — I do not want my service arbitrarily canceled just because I didn't see an email sitting in the bottom of my spam folder. Unless we're talking about some exceptionally expensive service here, any plausible price increase is unlikely to be more upsetting than total loss of a service that I apparently considered worthwhile in the first place.

IMO, the best thing is to grandfather people in. The next best thing is to just let people know what's going on, move them over and offer a refund if they are dissatisfied with the new plan.