Stat Changes
It seems very clear that you apply no stat changes.
This ability functions as a polymorph spell, the type of which is listed in the creature's description, but the creature does not adjust its ability scores (although it gains any other abilities of the creature it mimics).
There are no qualifiers here -- it says the creature does not adjust its ability scores, so it doesn't! The part of polymorph that you're looking at says that you
adjust its ability scores to one of these two sizes using the following table
This is exactly what the change self ability tells you to ignore.
Even though it doesn't use the word "adjustment" in the actual text of alter self, the word would generally mean any change to your stats. (And it would be a bit weird if shifting into a smaller form increased the strength of a Titan.)
Other abilities
There's nothing here to modify how polymorph spells work:
While under the effects of a polymorph spell, you lose all extraordinary and supernatural abilities that depend on your original form
Note the qualifier. You only lose the ability if it was contingent on your original form. In many places the DM will have to make a judgement call on whether that applies. But obviously, say, a manticore that shifts into human form can no longer use its Spikes special ability. The rule is intended to rule out nonsensical situations, so the change shape ability doesn't alter these rules at all.
Since this is effectively the creation of a new Dracolich, the following are true:
- It would fail without an Adult or Ancient Corpse
- It retains all the features of the new body
Here's my rationalization of those two.
By inhabiting a corpse of a newly (or heck, not so newly) slain dragon, the Dracholich is not simply acquiring a new body (though it is that too), it is in essence becoming an entirely new Dracolich. Thus it would need a properly aged corpse as conversion is not possible on younger Dragons (it would also need to be a true Dragon, and not a poser).
The other half of this is that the new body is tied to a specific type of Dragon and thus has it's stats/abilities etc and not the Dracolich's previous. It's a new Dracolich and not a rerun.
If I was going to use this feature in a campaign though, I would feel very free to experiment beyond these bounds. An ancient zombie dragon that has found itself bound to a wyrmling's corpse for reasons unknown could provide a very compelling plot possibility.
Best Answer
Short answer
The Spirit and Guardian Nagas both get a new body, their old body does not regenerate. It is up to the DM to decide where exactly this takes place.
Longer answer follows...
A rejuvenated Spirit or Guardian Naga forms a new body
This is not made explicit in the stat blocks of either creature, but it is mentioned in the introductory text to Nagas in general (MM233):
The Bone Naga introductory text also suggests that reinhabiting the same body (or at least skeleton) is a departure from the normal state of things:
Where the rejuvenation takes place
In the description of rejuvenation of some other creatures we find an 'anchor' near which the new body is formed, for example Mummy Lord's heart or Lich's phylactery - see the relevant MM entries. In the Nagas' descriptions of rejuvenation we find no such 'anchor', so the exact location where the Naga's spirit takes a new body is up to the DM. A DM may decide that the old body functions as this 'anchor', but this is not essential and certainly not suggested by the rules as written. In the case of Guardian Nagas, it makes sense for them to rejuvenate (respawn) near the "places and items of magical powers" (MM 233) which they are guarding, though in the case of movable objects that could get interesting!