Heroes 5.5 WarGame Edition - Tactical Edition.

That's a pity.

The concepts of "convenience" and "strategic depth" are not always compatible. It's convenient to be able to change a unit's upgrade at any time, but it doesn't create a sense of choice or its consequences.

Some flexibility will be provided by allowing the player to switch a unit's version from the base version to one of the alternative versions (i.e., only the "upgrade" will be allowed).

However, I suggest postponing this issue until the next stage, when the cities are being developed. In any case, this can be tested in practice before it is finally approved (or rejected). Perhaps this aspect will not be critical from a strategic point of view due to the high pace of the game (you won't have time to re-upgrade). In that case, re-upgrading can be allowed. There is a technical possibility to allow re-upgrading to any of the versions (including the base version).

So what's so critical about that?

Off the top of my head - an overpowered way to deal with slow neutral creatures by constantly running away and attacking with the hero.

The consequence of high speed is inevitably low physical stats. I think we can experiment with adding super-fast creatures among the neutral creatures when all stages are ready. And then we can see in practice how they will look. Fortunately, there is a large field for experimentation - the entire NCF is at our disposal.