Ok I have two choices, I can go swimming tomorrow and thursday or I can go to my sisters college and hang with her for the week... If you make a promise to do something even if something better comes along should you do it still?
Sometimes when push comes to shove promises have to be put aside, not in a case like this though, if you promised to go somewhere you should keep that promise. If the person you made the promise to would be okay with you changing plans go for it but if it's going to upset them I don't suggest it because you don't want to spend you Spring Break with someone mad at you.
A promise is a promise. There is no valid reason to break one.