i can't imagine the people interviewing you are going to even know you changed dates, and like was said, if it's a legit reason i think any reasonable person would understand. if it's for something kinda silly (ie, you wanna go to the britney spears concert in city X so you want to interview that friday) then probably not, but if it's to combine trips or something like that, anybody that was put off by that is probably someone you don't wanna work with for 4 years. also, i agree that ample notice is important to ensure the program can get another person in on the day you're cancelling.