Can you really debrief people to reduce the impact of them leaving? I've just been on a trip to help part of my organization debrief a retiring expert. Now, if I had put it to them bluntly, they were well aware that 20+ years of accumulated knowledge could not be conveniently extracted for them in a day - knowledge can't be tinned like tuna. However, it was certainly the fear of that loss that had prompted action. I had mixed feelings - helping out reinforced the misconception that it was feasible, yet surely it was better than nothing?
I tried to focus on assisting dialog with 'apprentices', rather than codifying anything. I also did a quick 'expectation' setting of why 'flavour' of knowledge could be transferred ('FASHEN' adapted from Snowden's model) - %ages represent how much of that flavour we could hope to transfer.
Facts - declarative knowledge. Yes. But most people know it in the field. 50%
Artefacts - documents etc. Yes. focus on structuring and context of when others should use. 25%
Skills - could be taught, but only over a long period 0%
Heuristics - rules of thumb, 'natural' theories. Depends on expert about how well they can articulate these. 5%
Experience - can be reflected in stories, say 5%
Natural Talent - non-starter 0%

Not a very encouraging picture. I was left with the feeling that what people value in an expert is not the depth of their knowledge but their ability to articulate just the right bit at the right time when asked a question. So its not the content at all, but the skill of retrieval that makes the difference.