When the Monster Manual's Manticore (page 213) uses its Multiattack, it can either combine a bite and two claw attacks, or it can make three tail spike attacks. A bite and two claws comes out to 19 damage, on average, and three tail spikes deal a combined 21 damage, on average. The DMG tells us to make CR judgments based on the more damaging option available to a monster, so the Manticore gets to use 21, but our spikeless Dragonne is limited to 19.
Looking at the Monster Statistics by Challenge Rating table (DMG 274), 21 damage per round is in the range of a CR 3 monster. 19 damage per round is in the range of a CR 2 monster. The cutoff is 20/21, so this is a fairly fine distinction we're making.
However, as described under "Final Challenge Rating" (on the same page), monster CR depends on other factors. The Manticore's HP falls in the CR 1/2 range, and its AC is in the CR 4 range, so its Defensive Challenge Rating comes in at 2. Our Dragonne's new damage per round is in CR 2 range, and its attack bonus is in the CR 4 range, so its Offensive challenge Rating is 3.
We get the final average rating by calculating the average of the monster's offensive and defensive challenge ratings: 2.5. "Round the average up or down to the nearest challenge rating," the DMG says. According to the text, we're free to treat the Dragonne as a CR 2 or a CR 3 monster.
But all this math only confirms what was pretty obvious in the first paragraph: Numbers-wise, we haven't changed the Manticore very much. We've decreased its average damage per round by 2, which in many cases wouldn't be enough to affect the CR at all.
Removing the Manticore's tail spikes may be an important change strategically: The Manticore is (roughly) as dangerous at range as it is in melee, but our Dragonne doesn't have a ranged attack. The DMG's guidelines for monster creation and CR calculation don't say anything about the difference between ranged and melee attacks, though, which means they don't affect Challenge Rating, the abstract, mathematically-defined concept, at all. Whether the Dragonne's reliance on melee has a calculable effect on how challenging it is in "real life" will come down to how you use it.
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.
Best Answer
This has very minimal impact on CR math
The easiest way to evaluate the impact of these changes is to evaluate how they impact CR based on the suggested guidelines for calculating CR for a custom creature.
From the Dungeon Master's Guide, Chapter 9 ("Dungeon Master's Workshop"), subsection "Creating a Monster", the following guideline is given for adjusting the CR of a creature, for Armor Class:
Because every single creature is having its AC increased by 1 with no other changes, this is equivalent to all creatures having their Defensive CR increased by 1/2.
The guidelines for Attack Bonuses are similar:
In the same way, this means the Offensive CR of attack-roll-based creatures has decreased by 1/2. Creatures that depend on Saving Throws for most of their damage won't see an Offensive CR change.
So for all creatures, Defensive CR is being increased by 1/2, and for attack-roll creatures, Offensive CR is being decreased by 1/2, with Saving-Throw creatures seeing no reduction in Offensive CR. For the former creatures, short of especially esoteric situations, the net impact of these changes is not likely to change a creature's overall CR. For the latter, a change of 1/4CR (averaging +1/2 and +0) may be significant at lower levels of play, but is unlikely to impact anything beyond the first few levels of play.
Impact on overall game balance
Battles are going to take slightly longer than usual. All creatures are going to hit with attacks slightly less frequently, which will mean that overall, they take less damage than usual. Healing becomes slightly more powerful under this ruleset, because damage totals are lower, which means healing represents a larger proportion of damage issued during a fight.
Damage will become slightly more swingy, with characters staying at their same level of Hitpoints for longer than usual.
Players (generally) benefit from this system more than hostile creatures
This mostly boils down to the happenstance of how damage sources work. Players are more likely to depend on Spells or Saving-Throw-based Cantrips than hostile creatures are, and Saving Throws are unaffected by these changes (being already "Defender Wins" as written). So those spells will stay at the same power level, whereas weapon attacks and Attack-Roll-based spells (which are more common among hostile creatures) will be negatively affected.