Use the DMG's guidance on modifying monsters
The DMG, on pages 273 through 279, includes guidance on modifying monsters and creating new monsters, including guidelines on how to calculate a monster's effective CR. This involves breaking down the monster's capabilities into a "defensive" challenge rating and an "offensive" challenge rating, then averaging the two.
Young red shadow dragons
A young red dragon has a normal CR of 10. By following the DMG's guidance on calculating its defensive CR, we see that:
- It has 178hp, which looks up to CR 8 in the table
- It has an AC of 18, 2 points higher than the expected AC of 16 for a CR 8 creature, so bumps CR to 9
- It doesn't have lots of damage resistances or immunities, so we don't treat it as having more HP for these calculations
For the DCR and OCR to average to 10, I'll assume the offensive CR is 11.
The single biggest difference adding the Shadow Dragon template to the dragon makes is to add lots of resistances that make it very hard to hurt (maybe only resistances it has in certain conditions... but then why would it ever stick around to fight you if it wasn't in advantageous terrain?) By adding so many resistances we justify raising the dragon's effective HP for the purpose of calculating its CR, as per Step 9 of the guidance on page 277:
- 178hp is CR 8
- Multiply HP by x1.5, for a CR 9 creature with many resistances, to give ~267hp
- 267hp is CR 14 (and AC 18 is appropriate for a CR 14 creature)
Averaging DCR 14 with an OCR of 11 gives us CR 13. It's plausible that the dragon's OCR was also given a slight bump because of the way its breath weapon can instantly kill adventurers now (the changes to damage type wouldn't normally make any change to its CR), which does make it meaningfully more lethal than the base creature - but OCR 12 would also average to CR 13 here.
At any rate, the change to the young red dragon's CR when applying the shadow dragon template as given in the Monster Manual seems consistent with the DMG's guidance on calculating CR when modifying/creating monsters.
So what of the Black Dragon Wyrmling?
This is a little harder to calculate since the wyrmling's stats are at the very low end of the scale and the DMG isn't super clear about whether or not CR changes should be steps on the table or full points, but we'll give it a go:
A Shadow Black Dragon Wyrmling's defensive CR:
- 33hp maps to CR 1/8
- AC 17 is 4 points higher so bumps CR two points to 2 (2 1/8) or 1/2 (two steps up from 1/8)
- For a CR <4 creature with many resistances, multiply HP by x2, so 66hp
- 66hp maps to... CR 1/2, again bumped by the higher AC by two points - to 3 (2 1/2) or 2 (two steps up)
And offensively:
- ~21 damage per round (assuming one use of breath weapon hitting two targets and two bite attacks over a three round period, as per DMG's guidance) maps to CR 3
- +4 to hit is appropriate for CR 3 so no change
Averaging those gives us CR 3. (Note that the base creature could be CR 3 or CR 2 depending on how we calculated it - as the MM gives it CR 2, that suggests the second method of stepping on the table rather than adding entire points is the better one.)
Fact is, the shadow dragon template doesn't make the black dragon wyrmling effectively that much more durable - it's maybe twice as hard to kill as it was before on paper, but against a normal adventuring party that change in effective hit points would probably only give it one more round of life.
The change to the breath weapon is probably the most significant factor in this case, since at the low levels where a CR 2 or 3 creature is an appropriate challenge, 5d8 damage could easily one shot a character and then they're dead outright. Bumping the OCR by a point to account for this gives us DCR 3 and OCR 4 which averages to 4 (3 1/2) - so 3 to 4 would seem a reasonable CR for the shadow black dragon wyrmling.
Of course, the DMG is quick to point out that the guidelines it provides are just that, and you may need to adjust the CR of a creature if it proves to be more or less dangerous in actual play. Many monsters in the published material have challenge ratings which are slightly off from what you would calculate from the DMG's guidance because of their special abilities and features that change the effective challenge they provide.
Yes; a shape-changed dragon can be cloned, and retain the ability to change back into a dragon.
When a dragon changes shape, its human form retains the ability to change shape (MONSTER MANUAL, p114). Therefore it would follow that a clone of the human form also has that ability. The Clone spell description states that the clone has the same abilities as the original (PLAYER'S HANDBOOK, p222).
An option for a possible adventure hook, if the above doesn't suit your game, would be the human clone of the dragon recruiting a group of adventurers to recover its remains. This would allow the clone to be slain, freeing its soul, and the dragon's original form to be raised from the dead or resurrected.
Best Answer
Since this is effectively the creation of a new Dracolich, the following are true:
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.