The techniques of Aikido change constantly; every encounter is unique, and the appropriate response should emerge naturally. Today's techniques will be different tomorrow. Do not get caught up with the form and appearance of a challenge. Aikido has no form - it is the study of the spirit. - Morihei Ueshiba
I remember, as I was coming up through the kyu ranks, it seemed to us that our instructor and his instructors were forever changing things up on us. They would tell us one way to do something, then a couple of months later (usually after coming back from a big seminar) they would tell us what seemed like a wholly different way to do the same thing.
I particularly remember several changes in how we were to practice kotegaeshi. That thing seemed to change with the phase of the moon.
This was always frustrating from the point of view of the student, but looking back at it from a little greater distance, It seems like just the way the thing has to be.
We are studying a huge, complex, and chaotic reality. You have to have some sort of form to put the thing into to study it, but you also have to understand that after you study one form of the thing for a while you will start to be subject to diminishing returns. You will need to look at the thing from a different point of view.
This does not invalidate the forms of the thing that you have already studied. It augments them... zooms in and emphasizes different facets.
Often neither the old or the new form of the thing will be the thing itself, but if you have a good teacher then the older and newer forms should sort of bracket the thing itself. The aiki always lives in the interstices between forms. But it is the forms that we use to outline the aiki-thing and study it.