What a person named Ruri posted on the unofficial board (that I translated without permission *thumbs up*)

Witch apprentice > Problem number 1. Since the memory of the summoning will remain with the main body, does that mean the information returns just for the summoning record?

The theories of the soul, ether/astral bodies are incorporated in Nasu Kinoko's world, so I think it's possible that it's making use of that system.
I don't have a detailed understanding of souls and ether/astral bodies, but I believe it works like this.
The soul itself is carrying the memories of the previous lives from older cycles and the current memories.
However, the current phsyical body can't cognitively recognize memories from a previous life, if I recall correctly. Because of that, it would be reasonable to infer that memories are split up between older memories (the memories from the soul) and memories that the body has experienced (current memories).
There was a conversation in Kara no Kyoukai that kind of supports that....I think.

/end
Cutcut.