From what I understand they are very particular about this. I mean if you have a major, unexpected life event they will work with you, but you can't just back in and out of class dates willy nilly. They expect your date of availability on your app to be accurate. If you have already accepted a class date and need it moved, certainly reach out to them, but again unless there are extreme extenuating circumstances I wouldn't expect them to let you change classes more than once. So if you have to back out make sure whatever new date you tell them you will be available is firm.