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.
I coded up a pretty spiffy popup menu system for small displays like phones and tablets. If you're on a small device, you get it automatically. You can force it on a large display by making the window narrow.