Glad it went well.
As a fellow migraine sufferer, I definitely vote for letting someone know ASAP. In my case, I have no car so getting from my home to a location involves a fair amount of public transportation. I'd rather know three hours ahead of time that things aren't working than get on my bus, get to the metro, wait for the train and then find out that you need to cancel. In the first case, stuff happens. In the second- I'd be pretty annoyed if it wasn't an emergency, and IMO a migraine you knew about several hours before the date would not qualify.