A conversion neck is one that can convert an instrument from one scale length to another with no other modification. A common conversion is 34" to 32". By overhanging the 20th fret, or leaving it off, you can use the stock bridge location on the body, just adjust for intonation. Such a neck will work on any standard bass body. A conversion from 34" to 30" requires overhanging two frets, or losing one or both.
This OLP Stingray was one of the first basses I ever modded heavily. I routed a battery and a second pickup cavity, and did all sorts of experiments on it, before returning it to it's original single-pickup configuration, but with upgraded active electronics and pickup, and a nice new pearl pickguard.