UNIT - Creature Balance + Visual Changes

Alt Hydras don't have poison; these abilities work completely differently. The only thing that "resembles" it is regeneration.

Alt-upgraded Hydras have acid blood, while upgraded Wyverns have animal poison. The mechanics of the abilities are different, but the essence is the same.

Moving Griffons to Tier 5 is less justified than moving Hydras to Tier 6; you could say that Hydras on Tier 5 is the biggest mistake made by the game designers. So, if you're going to start moving units, you should start with Hydras, not Griffons or Bears.

Moving Griffons was justified by at least two factors:
1) From a logical point of view. Griffons are mythical creatures with great power, perhaps even comparable to Paladins, while Monks and Inquisitors are just clergymen with minimal combat skills.
2) From the point of view of balance and game mechanics, Monks and their upgrades are rather mediocre ranged damage dealers; they only stand out because they are casters. However, Griffons do a very good job of dealing damage, even when they are on Tier 4 in the original game.

Regarding Centaurs, you can take a conservative approach and only swap them with Orcs (which are already close to Tier 2), or you can be bolder and move them to Tier 4 (Shamans to Tier 3). Or even to Tier 2. After all, a woman with knives is still a woman with knives, not the most formidable unit compared to Orcs.

Well, Shamans may be women with knives, but they are elders, much smarter and wiser than their fellow tribesmen, and they also possess extraordinary magical power; I think moving them to Tier 3 is a clear insult to them :D
But moving Centaurs to Tier 3 seems very logical to me, because ordinary Orcs can hardly cope with such a creature.

What did the Hydra remind you of? - one flies and has good mobility, the other barely walks and attacks in an area; besides, their poison is not the same.

It reminds me not of the mechanics, but of its poisonous nature; both are poisonous creatures.

Added 7 minutes ago

and those Bears are not teddy bears, but huge predators that used to eat Knight's horses for breakfast. So, if they are weaker than Knights, it's not by much - the mount is clearly stronger.

Bears are, of course, more powerful than Knight's horses, but horses are just a means of transportation for Knights... And Paladins generally have a very "close relationship" with Elrath; I think this can explain their position within their faction.