In the past whenever my work schedule has taken me elsewhere I simply inform the SP by phone - or email (if there are a few days before the appt.) that my schedule has changed and I am disappointed that we don't have the opportunity to meet - A quick to the point response is usually appreciated and has never been a big deal. I think that SP's get ah huge amount of cancellations and the more notice you give them the more they appreciate it as they can rebook with someone else. Just let them know as soon as you know you can't make it, and it shouldn't be a problem.