I disagree.
I always assumed the zombie template gives a Natural Armor Bonus that is added to the natural armor of the base creature.
In your example, the zombie giant turtle would start with +19 natural AC because the base creature is +19, but if the base giant turtle's size was Huge (for example), the zombie template would add +4 more natural AC, bringing its natural AC up to +23.
In other words, its shell doesn't get soft and squishy, it remains exactly the same but the Huge turtle also gains additional "natural armor" in the form of dead flesh that has no nerve endings and feels no pain - it's harder to hack through the shell and the lifeless unresponsive zombie flesh to do any real damage, hence the extra natural armor.
At least, that's always been how I understood the template to work.
Mechanically speaking, the CR of a zombie doesn't change much (compared to the base creature), though really big zombies gain HD (and natural armor) and so their CR goes up. Note that if you replace a really cool natural AC with the relatively puny zombie natural AC, then the CR should go DOWN rather than up since the monster is now much easier to hack to bits.
Given that the CR remains the same or goes up (size dependent), it makes no sense to do that when the zombie is actually getting weaker (at least in the cases of base creatures with decently high natural AC).
Since bonuses of the same type(other than dodge) don't stack, it would just count as having the higher of the two.
The skeleton template specifically states that it changes natural armor bonus.
EDIT: however, it does state in the zombie template that its natural armor bonus is based on size. So I would probably replace the existing natural armor bonus, and just say it turns mushy in the magically enhanced rotting process.
half-celestial, half-dragon and half-fiend specify that the natural armor improves.
lich specifically states it has the better of +5 or base creatures.
vampire states natural armor improves.
both skeleton and zombie templates state that it 'changes to' or simply 'is' the listed natural armor bonus.
Based on the wording of other templates in the same book, I would definately rule they lose existing natural armor bonus.
RAW, the Natural Armor Bonus is reduced to what is listed, since it specifically states "change".
RAI, zombies have mushy flesh, making it easier to affect, since the hardness of the flesh is reduced (due to lack of nutrients the body is supposed to provide).
Of course, their defenses do increase in other means; piercing and bludgeoning don't deal much damage because it doesn't affect much of the body, and since zombies have no feeling, it's like a poke with a knife or a slap across the face.
Animate dead does not enhance the rotting process.** spoiler omitted **
The skeleton template specifically states that it changes natural armor bonus.
EDIT: however, it does state in the zombie template that its natural armor bonus is based on size. So I would probably replace the existing natural armor bonus, and just say it turns mushy in the magically enhanced rotting process.
Fluff and 'reasoning' have no effect on the mechanics of the template being applied. It does what it does. In this case the application of the template sets the natural armor class to a number according to a chart contained in the template.
You know that doesn't fly for one second, as when you cast it they immediately lose all that AC, even if it's still fresh and not "mushy". This is completely and utterly "balance purposes". Dragon scales and turtle shells would not turn soft for extended periods of time if at all.RAW, the Natural Armor Bonus is reduced to what is listed, since it specifically states "change".
RAI, zombies have mushy flesh, making it easier to affect, since the hardness of the flesh is reduced (due to lack of nutrients the body is supposed to provide).
Of course, their defenses do increase in other means; piercing and bludgeoning don't deal much damage because it doesn't affect much of the body, and since zombies have no feeling, it's like a poke with a knife or a slap across the face.
Why not? The creature is dead. Just because it isn't decaying doesn't mean it's still providing nutrients. Mummies are a prime example of humans that are preserved for a long period of time and still having horrid flesh that falls off of the body.
Even with better "preservatives," akin to the real world's time period, it's still not going to amount to flesh that a living, breathing human being has; thusly, it is inferior. By the RAW degree? Not necessarily, though I'm arguing RAI, not RAW, which as I said is adjusted via its wording (and thusly balance).
The same concept applies here. That turtle corpse is malnourished and quite frankly nowhere near has the same level of strength that it normally would have. By your logic, a human being who has been starving for days has the same level of competence as a human being who is well fed.
Animate dead does not enhance the rotting process.
Are you sure? I'm not...
A zombie can be created only from a mostly intact corpse. The corpse must be that of a creature with a physical anatomy.
(emphasis mine)
This walking corpse wears only a few soiled rags, its flesh rotting off its bones as it stumbles forward, arms outstretched.
(emphasis mine)
You're right, the spell doesn't specify anything about rotting. But it does go from being "mostly intact" to "flesh rotting of its bones". So I think there is an enhanced rotting process, because a zombie(warning - opinion ahead) would never look like a mostly intact corpse, it would look like and be a rotting, walking pile of flesh and bones animated by the sinister forces oozing from the Negative Energy Plane. (/opinion)
I did not base my ruling of what the template does on the fluff(or flavor if your are not against it). I gave some flavor to my ruling, because that is what I do, when I rule - and I do rule! :)
I based my ruling of the zombie template on the exact wording of that template and the wording of some other templates in the Bestiary.
As for the turtle example:
Its shield tears free and falls off without the flesh to hold it in place. And the evil necromancer was astonished and quite disappointed when his apparently masterful scheme did not work and all that onyx was wasted. He looks at the pitiful creature that wiggled out of its shell and is now shambling about his laboratory and says: "Well, you live and you learn... at least until I figure out this whole phylactery thing."