Heroes 5.5 WarGame Edition - Tactical Edition.

Well, a speed of 3 in some cases might not be so bad, but the absence of units with speeds of 7-9 removes an important class from the game, in my opinion.

I agree with this; an entire class of units is being removed, units to whom "anything is allowed." Unfortunately, this class is incompatible with the established tactical landscape. Chess (tactical positioning) has its price.

In any other game where the emphasis is not on tactics, or where tactics involve other things, I would be all for having a class of units that are not limited by speed at all ("flying units"). But not here.

Here's a unit with a speed of 8. How do you defend against it, how do you protect yourself from it if it runs across the entire map? Its attacks are practically unstoppable; maneuvers become meaningless against it. Archers in a corner, surrounded on all sides – this is the result of having such units in the game.

Even for a chess queen, most of the squares on the board are only accessible in 2 moves. I can't imagine chess with pieces that can move to any square in 1 move.

For example, there are dragons, level 7. Growth of 1, with a castle, 2. Will the growth be 3 after upgrading?

Fortification will not affect levels 6 and 7.

Added 3 minutes later

Plus, to put it mildly, it's not entirely clear: if I upgrade a creature, and then I need the un-upgraded version for some reason (let's say I think it's more useful in the endgame), how do I downgrade it?

That's where the significance of the choice lies. If the basic version is needed in the endgame, and the alternative version is needed for breaking through, then either you divide it into 2 stacks, or you choose one or the other. However, this question concerns not so much the alternative system itself, but rather another point: whether to allow re-upgrading.