About changes and/or combining Hero Specializations.

Correct me if I'm wrong. Let's take Necropolis as an example; it has two specializations for creatures. I can only create a specialization for, say, Liches, by overwriting one of the existing ones, and it's impossible to add it as a third creature specialization, right?

That's correct.
To create a specialization for Liches, you'll need to either modify the specialization of an existing Necropolis hero, such as Orson.
Or borrow from another Faction, for example, "steal" the Orc specialization from the Orcs and systemically swap it with the Lich specialization.
You can also create a "non-standard" specialization, like the Witch specialization for the League or the Wyvern Tamer.
You can "steal" the Centaur specialization from the Orcs (its difference from the regular one is that they also receive an attack bonus at the beginning of the battle, proportional to the hero's level) or the Tier 1 Gnome specialization (which, in addition to stats, also buffs HP) 🤔

The main thing to remember is two simple rules.
1) If you simply modify an existing specialization to another Tier of your Faction, there will be no problems.
2) If you take a "foreign" specialization for Creatures, for example, from another faction, don't forget to give them another specialization in return, or remove their Hero from access.

Added 2 minutes ago

Are these conclusions definitely verified?

Because when I was testing and adding a third tier of creatures, I thought that the creature specialization was hardcoded to specific creature IDs.
But when I started adding them, it turned out that the specialization also works for new creatures, if they are creatures belonging to the same Faction and the same Tier.
For example, Razaz buffs Golden Golems with his bonuses; these are new IDs, but they belong to the Academy and are also Tier 3.

I was also thinking about this and want to test it. (This is also an important detail for me for future modifications)
I think that the connection sounds like "N Creature and its Upgrades," i.e., the connection goes to the base version of the Creature and then to all those listed as its Upgrade 🤔

Added 8 minutes ago

I hope this works for you, but I was disappointed with this method after a mod that shifted Griffins to Tier 6; then, in one scenario, I had to fight 40 Griffins, not level 4, but level 6. The mod went to the trash.

Yes, there is that. Moreover, we have already noticed that such changes also affect the Random Creature Generation. It seems that "random generation" is also more tied to specific creatures, although then I have questions about how it could have worked correctly in mods like ЕВА (mods where completely new creatures were added) 🤔

There are no problems with the method. Since my modification is currently exclusively for Multiplayer/Co-op. The playability of Campaigns and Scenarios "died" already at the stage when we removed the Orcs. 😅

Added 4 minutes ago
In any case, I'm glad that this topic didn't die in the title. Who knows, maybe we'll find something else later.