Well, essentially, I'm suggesting a recoloring and a swap. Keep all abilities and stats the same, but replace Dragons with Messengers, and Messengers with Knights. There's no need to even repaint anything – just swap them. If Zombies are getting abilities from other units, what's the problem with giving Messengers the abilities of Dragons, and Knights the abilities of Messengers? Although, there's already a mod for Knights – just include it and update the stats. And Dragons can guard various crypts and dungeons – that's where they belong.
Once again, I'm not against such a swap in principle, but not in this mod, where the units are already well-established in their roles, both in terms of features and atmosphere. It's just that a newcomer might not understand this established order, especially if they have little experience with battles in Heroes of Might and Magic. But it exists nonetheless. Therefore, I advise playing for a few months with what's already available, and then suggesting radical changes.