For some reason I didn't see this until now...
Yes, a 24.75" scale length guitar will have a different spacing than a 25.5" scale length.
For the rest, I sense some confusion...
If you have a 21 fret 25.5" scale length guitar, you can put a 24 fret neck on it IF the 24th fret lands 19.125 inches from the nut, and 6.375" from the saddle. If it's anyplace else, your frets will be in the wrong place.
The length of the neck doesn't determine the scale length - the distance from the nut to the saddle does. So if you take a 24.75" scale length neck, and you put it on a guitar in such a way that the nut is anyplace other than 24.75" from the saddle, your frets will be in the wrong place.
You don't move the bridge to compensate for tension - you move it to get the scale length to match the fret positions. If you have too much tension on a guitar, your scale length is fixed - so the only way to reduce the tension is to get lighter strings.
If you decide to swap one neck for another with a different number of frets, you open up a whole can of worms... if the scale length doesn't match fret position, you will never be in tune (and this is different from intonation - no saddle piece adjustments will help). If the scale length matches fret position, you need to consider the placement of the pickups. For example, on a Strat (which has 21-22 frets depending on origin) the neck pickup is where the 24th fret would be - it was placed there for a reason. If you replace the neck with something custom engineered so the 24th fret falls right where the 21st would have been - giving you a 26.324" scale length if my math is right - the pickups will now be located closer to the bridge than they were. This will make your tone "twangier".
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