What is unchanging?

PIt seems that the physical world is. It also seems that every aspect of the physical world is in flux. It's all changing and all passing away. Then there is the experience of the mystic of the apparent unchangeable and in unchanging ground - consciousness itself - in which the world arises. 

Experiencing the unchanging ground now, I seem to have the same experience of the same thing that I experienced two days ago, or two decades ago for that matter. I recognize it instantly as the same - seemingly in a different way from recognizing an old friend I haven't seen for two decades - my friend seems familiar, but I have to struggle to recognize the face, to bridge the physical changes over the passage of time.

Then there's another friend who has changed little, but they've changed. You fall back into an easy familiarity and its as if the years between have never happened.

And the friend I saw yesterday. Had a haircut and changed his clothes, but has not changed in any essential way since then.

In the longer term cases, given enough exposure now, the memory of the old part of the old friend is gradually altered so that it becomes difficult to remember the two decades past version and you need a photo or video to bring that one to mind.

So is the unchanging ground so unchanging? Nothing seems to happen there, nothing in my experience has apparently changed. But how reliable are my experiences in this? I recognize it immediately and unconditionally as the same consciousness I experienced before, but how complete is my experience of it now, and my memory of the experience of it then, that I can build an entire cosmology on its unchanging nature? What I experience now, I immediately remember experiencing then, and it seems that nothing has changed. But all I can say with certainty is that I haven't noticed any change.