The way I see it, you basically have two options here:
1) Get a neck that's the same length as your 20-fret one, but with a fretboard extension, so that the fretboard hangs over the body to give you the extra 4 frets. The downside to this is the cutaway designed for a 20-fret neck will likely not give you very easy access to those upper frets.
2) Get a longer neck and shift the bridge closer to the neck to make sure it intonates properly. This will give you access to all the frets (as much as you had before, anyways), but since you will have shifted the the position of the scale relative to the body, the pickups will now be much closer to the bridge, changing the soud you get from them. Maybe it will be to your liking, maybe not. You could, of course, route new pickup cavities to compensate for THAT if you want your pickups to sound like they did before.
Seems like no matter what, it'll be a compromise and you'll have to decide what you want to sacrifice. That's my take on it; I'm curious to see what others have to say.
Left-handed guitar right-switch-hand prodigy.