I've had better luck giving the snake food on schedule every week. A large part of the time, if it's not going to be insanely stubborn, it will take the new food item in a few weeks. I offer the new item first. If it's not accepted, then I offer the snake what it's used to.
I have had huge success with waiting just one day after the usual feeding day, and scenting the rat with mouse urine. I personally don't think letting them fast helps much--they are creatures of habit, and they're used to going very long stretches without food. If they go into fasting mode, they can outwait you, and they will starve themselves to death if they aren't inclined to accept the prey you provide. This has happened many times with imported ball pythons.

I have some frustrating ball pythons who will refuse rats 80% of the time. They turn up their nose completely. When offered mice, they take them right down. I keep offering rats, as every once in a while they eat them. Since the goal is to get them to eat and keep putting on weight, there's no sense in insisting they take rats if they'll only take them once every 3 weeks.