Well, the 12th fret needs to be exactly half way between the bridge and the nut when the neck is installed, likewise, the 24th needs to be exactly 1/4th the scale length from the bridge. If you try to just bolt a longer scale neck onto the guitar, wouldn't the 24th fret be in the same location as it was for the shorter neck, meaning it would then be too close to the bridge? And all the other frets would be in the wrong place too, wouldn't they?
Unless you very precisely redid the neck pocket to make the new neck line up in the right place, and since you'd need to make the longer neck move away from the bridge compared to where the shorter neck was, there'd be a gap that you'd have to fill in. Doesn't seem to me like it would be easy or advisable.