HEADS UP: Loading a pre-existing save should work just fine (Drive Forms and Growth Abilities start at scratch) but to be safe, don't save OVER your file, save on a new slot.
Main story campaign was played and beatable on Standard mode. I don't see Critical being remotely possible. Let me know if I accidentally broke something that prevents progress. Remind has yet to be tested but will be.
Play all of Kingdom Hearts III as Kingdom Hearts II's Sora: his model, animations, combat, magic, physics, Drive Forms and menus, rebuilt from your own copy of KH2 (Kingdom Hearts HD 1.5+2.5 ReMIX on PC). Nothing from KH2 is shipped with the mod; it reads your install the first time KH3 starts.
Combat
KH2's own attack choice, combos, finishers, Combo Plus / Air Combo Plus, aerial and ground abilities (Slapshot, Dodge Slash, Explosion, Aerial Finish, Guard Break, Retaliating Slash, Aerial Recovery...).
KH2 Guard, Dodge Roll, Quick Run, High Jump, Aerial Dodge and Glide, earned through leveling Drive Forms.
Magic
KH2's Fire, Blizzard, Thunder, Magnet and Reflect, with their KH2 casts, effects, and combos. Water and Aero spells still here too! Magnet and Refect unlock and upgrade alongside Water and Aero respectively.
Drive Forms
Valor, Wisdom, Limit, Master and Final Form with their KH2 models, movesets, magic, form levels and growth abilities, unlocked through KH3's story. KH2's Drive gauge, Drive Recovery items and the Drive command replace KH3's Links.
Anti Form replaces KH3's Rage Form: KH2's Anti model, moveset, sounds and rules, activated KH3's way.
World outfits: KH2's Halloween Town Sora in Monstropolis and Space Paranoids Sora in San Fransokyo, forms included.
HUD and menus (buggy)
KH2's command menu drawn from KH2's own art, with each world's KH2 menu style, reaction prompts and the L2 situation command list.
KH2's HUD face in the HP ring, changing with forms, outfits and damage.
KH3 kept where it should be
KH3's attractions, rides, team attacks, swimming, the ship and sky battles, skydiving and the final battle's special phases run KH3's own controls on the KH2 model.
KH3's cutscenes use KH3's KH2-outfit Sora, so the story looks right.
Requirements
Kingdom Hearts III (PC) and Kingdom Hearts HD 1.5+2.5 ReMIX (PC) installed.
Works with existing saves; Drive form levels start fresh on a save that never had the mod.
Tune or switch off individual features in features.txt in the mod's folder.