Never (ever) break plans that you have already made because something "better" has come along.
Follow through is something that I admire and really respect. If you tell someone that you are going to do something then you need to do it. Everyone is worthy of your time and advance plans need to be respected. One of my biggest pet peeves is to say that you will do something and then you flake out. I do have one caveat to this statement...if you have plans with good friends and if you get offered backstage passes to the Brad Paisley concert, then plans can be postponed and/or cancelled. Good friends will understand as long as you are honest.