Still, if you have sufficient amount of "event" data, a brain like structure, even if virtual, could possibly fill in the missing parts. Current research on human brains and memory shows that most of us don't remember as much detail as we thought and that the brain is capable of "filling in the blanks" to give the impression of a detailed and vivid recollection.
The idea of "essence" is philosophically suspect anyway. Who's to say that the step between Yui and a fluctlight is really that far? It is a fictional world after all.