Ive found this to be a bit of a problem - Ive got various campag hubs all with the same 12mm axle that have slightly different freehubs. They all look the same but are not interchangeable. hence why ive got 2 or 3 spare unused freehubs !
I've just recently taken off an 9/10 speed freehub body and replaced it with an 8speed the part you have to change to make it work is the long sleeve spacer inside there is a 2mm difference between 8 & 9/10 speeds