Updated for 1.3.0 version. Will only be supporting full, stable RELEASE or BETA builds, not ALPHA builds, going forward.

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All it does is remove Horse upgrade requirements for ALL FACTIONS starting units that require a Horse. War Horse still required and not removed
requirement.

The units that were changed in this version are the following:
aserai_mameluke_regular
aserai_youth
battanian_scout
imperial_equite
khuzait_tribal_warrior
khuzait_noble_son
druzhinnik
sturgian_hardened_brigand
vlandian_light_cavalry
vlandian_gallant

Optional variant of this mod mod removes BOTH Horses and War Horses upgrade requirements for ALL FACTIONS. No Unit Stats fixing in this version.
This optional mod alters the following units to NOT require War Horses upon upgrading:
aserai_mameluke_cavalry
aserai_faris
battanian_mounted_skirmisher
battanian_horseman
imperial_heavy_horseman
bucellarii
khuzait_horse_archer
khuzait_lancer
khuzait_torguud
druzhinnik_champion
sturgian_horse_raider
vlandian_vanguard
vlandian_champion

So, if any other mod edits the above, then this optimal mod would conflict. If not, then not. I tried as much as possible to make this modular. So I don't touch any infantry, archers, mercenaries, caravan guard units, just the starting FACTION ROSTER units that require Horse or Warhorse upon upgrade. These are the ones I found. If I missed any, please let me know.

Credit: Thanks to Maelstrom89 for the explanation and SomeDudeOnTheW3B for tutorial

Automatic Installation: Vortex installation SHOULD work. Please let me know if it does not.

Manual Installation: Unzip the folder and drop it here, in the following installation path:
\steamapps\common\Mount & Blade II Bannerlord\Modules
Enable the mod in the game launcher.

Compatibility: Compatible with all mods that don't change the specific FACTION units in the spnpccharacters XML file that are mentioned above (aserai_mameluke_regular, etc.). Should work with save-games. Conflicts with any mods that change the specific FACTION units in the spnpccharacters XML file.

Optional mod that adds the War Horse upgrade requirement waiver for ALL FACTIONS. It CAN be downloaded via Mod Manager like Vortex. Still, since the name is different. Be sure to uninstall the MAIN mod though! It's just a different variant of the mod.

Check out my Khuzait Units Fix mod

Take care! Cheers! Kudos! :) and enjoy your time in Calradia! :D