It is a strength of science that scientific theories undergo change - because that change is driven by observations of nature.
Change for its own sake would not be a good thing, but in science, theories change when it is shown that they do not conform to observation.
Theories are never fixed and undergo change and are refined through the years.
Theory change is a community process of feedback, experiment, observation, and communication. It usually involves interpreting existing data in new ways and incorporating those views with new results. It may depend on a single definitive experiment or observation to change people's views, or it may involve many separate studies, eventually tipping the balance of evidence in favor of the new theory.
The process may take some time since scientists don't always recognize good ideas right away, but eventually the scientific explanation that is more accurate will win out.
This process of theory change often involves true scientific controversy, which is healthy, sparks additional research, and helps science move forward. True scientific controversy involves disagreements over how data should be interpreted, over which ideas are best supported by the available evidence, and over which ideas are worth investigating further.
For further reading, see Berkeley.edu - Even theories change.