Significant changes to a setting?

[Icefalcon]

It all depends on how often you use dragons in your games. If they are very commonly used, then yes, it would be a big (but not huge) shift in power. If they are uncommon but not rare, this would not change much of the dynamics. I, myself, use more powerful dragons then what is presented in the material. When they show up as enemies, it should be causing the players to shake in their boots. When they show as allies, it should cause great relief. I feel that the current dragon stats in the books do not capture this dynamic well.

As for how to handle the switch over, I would just add them in as you want them. Large powers in the area (such as the Coalition) would not be displaced by the increase of power. They have enough firepower for the upgrade to be of no noticeable difference except a little more loss of assets. For smaller groups (such as the player group) the power will be almost overwhelming, as it should be when dealing with dragons.

As for the hatchling stage, I would not eliminate it altogether but instead make it happen off screen and then maybe accelerate the process of growing to full size take only a matter of weeks or months instead of decades. That way, if it was you are shooting for, the players will be unable to play a hatchling in your games.

[Ectoplasmic Bidet]

As Icefalcon said, it depends largely on how often you use dragons in your games.

There would be few outright major setting changes that would need to be made to accommodate your wishes. Lazlo probably wouldn't exist[and thus neither would New Lazlo] or it would be vastly changed without the early and marked presence of Plato.

I'm curious as to how you're making them even more powerful. Adult dragons already brush up against low to mid level deific power and the ancients of several species can negotiate with most gods from a position of equal or greater power.

[Ectoplasmic Bidet]

Plato, God of Magic and Philosophy?

[Icefalcon]

Plato, God of Magic and Philosophy?

Demigod maybe.

[Icefalcon]

As for the dragon NPC's in settings, I would leave them as is. They just hide the fact that they are dragons. Plato is still there, they just don't know he is a dragon.

So far, it sounds like a straight swap out for the current dragons and the new ones should be simple. There is not much of a change to the balance of the world if they are uncommon.

[Icefalcon]

Anytime