If the 24 fret neck is built for a 25 1/2" scale instrument, it will (should) work. Any time you swap necks, you might have a slight intonation adjustment due to the heels being slightly different from manufacturer to manufacturer, but if you take that 24 fret neck and measure the neck from nut to 12th fret and it's 12 3/4", then it will (should) work...dig?
But these 24 fretted necks typically have an extended tongue fretboard, the heel would be at the same place a normal 22 fret, 25 1/2" scale would be. If they make the whole neck longer, including the heel, then it probably wouldn't work, because you would be moving the nut further back and THEN throwing off the scale length and intonation at the same time. The other issue is if the extended tongue fretboard will interfere with the placement of the neck pkp, if it will actually hit it.
Basically, the amount of frets doesn't matter as to whether it will work for a particular guitar or not, it's all about the scale length the neck was built for in the first place...dig that?
For instance...you could have a 21 fret, 22 fret, and 24 fret neck, and all would work on the same guitar body IF:
the body had it's bridge placed at the proper location for a 25 1/2" scale length
AND the neck was manufactured to be a 25 1/2" scale neck.
You work it out by (as Saber already stated) ...by doubling the distance from nut to 12th fret. 12 3/4" X 2 = 25 1/2".
So for that scale length to 'work out' and be able to intonate properly, the bridge saddles need to fall under the strings at the 25 1/2" mark (give or take a few 1/16" for intonation purposes)
From what I've seen, overhangs are not that common.
And what you say about necks being interchangable if scale length of the neck, and the bridge placement on the body is *not* true.
The heel has to be at the same distance from the nut too. And it usually isn't, since it usually is right under the last 2 frets. So a 24 fret neck would normally have the heel much further down the neck than a 21 fret, meaning the scale length, and thus also intonation will be off.