Dark champions of Necropolis.

but you said that all stats and abilities would be preserved from the original -_- so, flight should also remain.
I just don't want any changes to the balance, just the creature model.

Then delete it too, then nothing will change at all. In general, you need to make a 2nd version of the mod where only the models are changed, because there may be many people who don't want to change the balance (and it's not a fact that they will all want to just delete the unnecessary and be happy, like you, Imaris).

150-160 to 190-200

In my humble opinion, that's too much.

It's better to improve the messengers - they are just a disgrace. At least reduce the price of buying them.

They are worth the money, not the worst unit in the game. And it's not difficult to balance the mod, you can easily handle it yourself.