The only known reference to the Law of Change by Alice A. Bailey (AAB) in her voluminous writings and those given to her by Djwhal Khul (DK). Links on these pages of the Laws of Alice A. Bailey are to the online books at the Lucis Trust website.
What is a law?
… a law is but the spiritual impulse, incentive and life manifestation of that Being in which he lives and moves. A Treatise on White Magic , INTRODUCTORY REMARKS
A law is an expression or manifestation of force applied, under the power of thought, by a thinker or group of thinkers. Discipleship in the New Age, Vol. I , SECTION ONE – DISCIPLESHIP IN THE NEW AGE – Part 2
… a law is in reality the effect of the life of a greater entity as it encloses a lesser within its living processes.
Esoteric Healing , Part Three – The Fundamental Laws of Healing
… a law is but the effect of the continued intelligent activity of the Life aspect as it works in conjunction with matter. Esoteric Psychology – Volume I , 5. The Rays in Cyclic Manifestation
A law is only an expression or manifestation of force, applied under the power of thought by a thinker or group of thinkers. Esoteric Psychology – Volume II , II. The Seven Laws of Soul or Group Life – Part 5
A law presupposes a superior being who, gifted with purpose, and aided by intelligence, is so coordinating his forces that a plan is being sequentially and steadily matured. A Treatise on White Magic
Quotes I found below:
1. That the Law of Change governs the Hierarchy just as it governs Humanity.
Although AAB only mentions the Law of Change once in her writings, she has a lot to say about “Change” and “Changes” – see quotes below.
Changes are now taking place in the egoic lotus, and the petals are unfolding, that unfoldment being partially dependent upon the vibration in the spirillae and their awakening.
Therefore, my brother, from the angle of the searching disciple, this first hint (in one of its deeper meanings) provides you with three lines of thought or of meditative reflection: Evolutionary Change, Reorganisation, Group Responsibility.
Changes constantly occur.
Changes in curriculum and in techniques are being made by the Hierarchy; the adaptation of old methods to modern needs and to more highly developed men is under way.
Changes mean nothing unless they are the result of new vision, for if they emerge out of a criticism of the past and of what has been done, they will prove useless from the angle of the spiritual life, no matter how useful they may be from the angle of the organisation.
Seek, my brother, at any cost, to be alive and eager for the future. Hide never behind the thought of past achievement or of achievement in some future life; learn to recognise opportunity when it stands before your thought and be ready to change the stable rhythms of a high grade and adequate personality for the eager forward looking attitude of a world disciple. Changes will then come because your inner attitude has prepared the way.
Change is ever needed but the method changes itself from the constant variability and mutability of the Mutable Cross to certain changes of a major kind which are brought about by a more permanent and directed life tendency.
The little one upon that little world responded slowly to that light until today the little world of men begins to throb in rhythmic unison. Changes are wrought.
Death can, however, be more than these things, and can be met in a different way. It can be made to hold a definite  place in life and thought, and we can prepare for it as something which cannot be evaded, but which is simply the Bringer of Changes.
Forget not, brother of mine, that this is not the first time that this has happened. Changes were made in Atlantean times; the door of entrance for the animal kingdom or for animal man into the human kingdom was stopped.
Changes have however been made, and a Master can now pass on to this sixth Path directly and without entering the deva evolution.
3. Changes in the type of egos or souls which will take incarnation during any particular ray period.
Here, my brothers, is a simple statement of the facts which must be grasped by all of you who seek to participate intelligently in the Wesak Festival and who are anxious to act as transmitters of the spiritual energy which will, at that time, be poured out to suffering humanity. This Wesak Festival in 1941 can prove to be a “Changer of Conditions” and a major turning point in the life of mankind if every spiritually minded person can bring himself to the needed point of selflessness, disciplined purity and resultant receptivity.
You are welcome to post your comments and understanding of this law.