Still, since she ingrained that reflex into his mind he should be able to do it again, if need be.

 

But did he retain that specific reflex when he has not excersied it in 3 yrs+? The guitar thing is still there b/c he regularly practices, but he hasn't touched Necromancy in a while now.

 

She told him the relevant information form the Word concerning the Darkhallow. Saying he doesn't remember this is like saying you don't remember your last night drinking with your buddy before he got into a car crash.

And Dresden already knew the theory of necromancy. Whether Lash supplied him with the knowledge to apply it or he came up with it by himself he would still retain this knowledge as it is his own.

Dresden has lost no knowledge.