You should change your belief in a rule when you discover the rule is false, notice limits to it, find a better rule, or discover and live the principle that it came from. The best way to ensure you have the best set of rules is to always live the rules you believe. By doing so, you will quickly gather evidence for or against the rule, learn its limits, and gain motivation to exert greater effort in finding something better.
One barrier that prevents people from living principles is their willingness to change their principles in the moment as a result of an increase in cost for the living the principle or increased desire for something that violates it. You cannot be a principled person while violating your principles when they are tested. The benefits of living by principles are gained precisely by living them at all times and in all places. Principles can change, but you should view those changes as intentional and outside of the scope of what happens as a part of daily life. Changing your principles isn’t something you do when you are in the middle of something else. They should be changed through intentional thought and during set aside time.
Shifts to character qualities are done through looking for and finding examples of people with better character than you. You can assess your character qualities, choose qualities that are better than your own, and intentionally apply them in daily life. An effective strategy for making these changes is given in the author’s book Joy on Purpose.