CR ignores versatility
I will preface addressing the two "extra" abilities the Gold Dragon has over the Red Dragon by saying that the CR calculation described in the DMG ascribes no value whatsoever to versatility. The main way that you calculate the effect a monster's battlefield impact has on its CR is its damage:
If a monster has different attack options, use the monster's most effective attacks to determine its damage output.
Thus if I give a monster 2 attack options that allow that monster to deal damage in slightly different ways, each of which will be useful in a specific situation, but overall they are equally effective, the monster would have the same CR as if it only had one of those options.
The DMG does provide a table of special traits that affect CR, and suggests this should be extended for custom traits - but those only ever affect CR if they increase the capabilities of the creature without opportunity cost, not if they simply increase a monster's options.
Another example of this is the half-dragon template from the Monster Manual:
Challenge. To avoid recalculating the creature's Challenge rating, apply the template only to a creature that meets the options prerequisite in the Breath Weapon table below. Otherwise, recalculate the rating after you apply the template.
Here there's a specific CR under which adding the Breath Weapon to a creature doesn't matter, because it's assumed its other attack options will deal as much or more damage, and giving the creature another option doesn't change the CR.
So - CR ignores versatility, and only cares about raw power.
That being the case, I will now demonstrate that these two features are side-grades to the rest of the dragon's powerset - and thus that CR ignores them. Beyond that, I do genuinely think the majority of the time in a fight, the dragon will be better off not using them.
Change Shape is almost always a downgrade in combat
Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form.
The dragon can turn into humanoids or beasts only; no weird aberrations that have absurdly powerful situational abilities, and no "noble genies".
The highest CR published Beast is CR 8 (T-Rex). Prior to the publication of Mordenkainen's Tome of Foes (i.e. when the Adult Gold Dragon was published), the highest CR humanoid was CR 12 - now there are a few above it.
As such, the dragon will be using this ability to turn into things less powerful than it is, since only the CR 17 and 24 versions of these dragons have this ability; yes it may gain some situational powers, but as previously stated CR ignores versatility.
It also retains its HP - so unlike other polymorph-like abilities, it cannot use this to soak damage.
Finally, there is nothing in the ability that would suggest it forms the equipment of the creature it changes into.
Weakening Breath is usually not that good compared to Fire Breath
The gold dragon must choose, each time it has an opportunity to use its breath weapons, which one it wants to use.
Breath Weapons (Recharge 5-6). The dragon uses one of the following breath weapons.
The Adult Gold Dragon, for instance, can choose to do one of the following things in a massive area:
- Fire Breath: Do roughly 66 damage (since most creatures will not succeed a DC 21 Dex save); This will kill most creatures outright, and deplete the HP of most high level PCs in 2 clean shots.
- Weakening Breath: Give creatures in the area a Strength save to avoid a minute of disadvantage on Strength attacks, checks, and saves.
Weakening Breath has a big "weakness"; If the creatures being targeted are a threat because of their Strength-based attack rolls, they are likely to have high Strength and perhaps Strength saving throw proficiency. Target a high level Strength-based Fighter with this, and they will have about half chance to succeed. The ability redeems itself a little by causing Strength saving throw disadvantage, thus making it harder to throw off once it sticks, but it has to connect first.
By using this option, you're giving up almost certainly about 66 damage on this creature, since if it's strong in Strength it's likely to be weaker in Dexterity saves. And that is almost certainly half its HP or more.
As well of that, it's important to remember that this is an area attack; Dexterity-based creatures or spellcasters in the area won't care about Strength disadvantage; but they will definitely care about damage - likely more than the Strength-based creature.
Sometimes Weakening Breath will be useful - but most of the time Fire Breath is better. And CR doesn't take this into account because again, it ignores versatility.