If the teacher finds he's uncomfortable with an already stated and published rule, he should be very cautious of changing it immediately. Depending on the maturity of the class, changing the rule mid-term could cause confusion. The best thing would be to wait until after the term is over. There are many factors involved with being an effective manager. Having set rules, procedures, and consequences is one factor.
Positive discipline is another one.
Positive discipline is emphasizing the use of positive reinforcement. It's more effective to remark on the good things the students do than the bad. If you comment only on the unwanted behaviors, the students will come to the conclusion that these acts are the only way to get your attention. To do this a teacher must always be aware of everything that is going on in the classroom. If a student feels he can get away with something while the teachers back is turned then he will be tempted to try it.