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 guy is obviously a master with a belt sander, he makes this look easy, and he does it fast too. I used to use a belt sander for neck shaping, but there are a number of difficulties with that, it is easy to screw up, and the dust flies everywhere.