What would you consider good reasons to change a system of martial practice?

There are prerequisites to this question and the answers. A prerequisite or set of prerequisites:

A. A certain amount of experience in teaching, practicing and applying the system in question.
B. A certain amount of proficiency of "making it work" in the appropriate contexts.
C. A certain age of the proponent of the system.
D. A certain number of years in the system.
E. The ability to answer the question, is it needed? Is it beneficial? Does it work?

This is the minimum. If you cannot answer it an affirmative then you don't need to change the system. Change simply for the sake of change is not good for the system. It may be beneficial for an individual in practice only but for the system and especially for teaching of the system do not change it unless you can address all the requirements and specifications.

Most of those who made changes to karate had spent the time, effort and gained reality experience before even considering a change. In a lot of cases the changes were geared more toward a specific philosophy of the practitioner - the system adherent. Often those changes are of a subtle nature and none of the fundamental principles of martial systems changed - they can't or they lose the status of principles and undermine the foundation of a working system.

When you do reach a point where you meet or exceed the prerequisites then you still have to vet the change. In a short version the following questions need to be answered before trying to change the system as a whole by its parts or specifications.

Specifications on Change:

1. To add a feature.
2. To fix a broken feature.
3. To improve on a feature.
4. To optimize the system for a specified and accepted feature.

In most cases the addition of a feature is a more esoteric change that is governed by:

"The work of art is always produced by a certain person in a certain time and place, and it is always related to its author's other works, his contemporaries, his sources and traditions, his intellectual, political, economic, and aesthetic climate. Background materials that can help the modern reader grasp the ideas or catch the flavor of a literary work of the past serve a valid and necessary purpose." - Damon Knight, "Creating Short Fiction: The Classic Guide to Writing Short Fiction."

To bring that into a direct form I would say that the person instituting the change only does so because the current time and place as to the current customs and morals of the time and place; the persons intellectual level; the climate of the times; the persons background and the ideals of the society as to ramifications if the change or addition of the feature is validated.

This would also apply to all the four specifications because it would be a critical matter. If a feature that was once valid and worked suddenly begins to fail it must be determined that the failure is due to the reasons above. If the failure is indicative to an individual regarding such as application at the moment, etc. all features of a system fail in this instance. It must be determined that the broken feature transcends the individual and is indicative of all applications at every moment. A very difficult spec to determine.

Lets step up and add both to any and all improvements. It must be necessary to all the reasons already provided. It applies to the optimization of the entire system.

It also must be asked, does this require change or simply dropping from the system entirely. This last is important for the moment you lose it you have changed the original classical intent of the originator's intent in creating the system. This is neither a good or bad thing but rather a step taken with extreme caution.

Change is inevitable, it is nature's way and it governs all life and all living things on Earth. Don't enter into this lightly or quickly.

p.s. this part is only provided because it is usually the reason most change the system or amend the system; to justify commercial gains and the pronouncement of mastery and the highest rank of the martial arts - 10th Dan.