As the title says, everything you can imagine has had its Battle Level changed to 99. Every single world and boss, no matter where in the story. However, bosses' HP does not increase automatically when changing the battle LV, so I changed their HP manually. You can check the images section for a boss HP update breakdown. Data Luxord is unfortunately unchanged as he relies on time, not HP (Keyblade Graveyard Luxord is fine). Battlegates are actually unchanged as well since I don't know how to edit their Battle LV, but they're pretty minor anyway.

Of note: you can install this mod anytime during your playthrough and it will take effect (same for uninstalling).

Just how bad will difficulty be now?
If you install this mod and play on your 100 % save, with a LV 99, 100 STR/MAG/DEF, Ultima Weapon Sora... Actually, it will not be that bad. Postgame Sora's damage output is simply ridiculous. It'll only make your postgame have mobs worth fighting (not Hell&Hell at all). And even the Data Battles with slightly buffed HP won't make much of a difference (beware for Data Young Xehanort though, now you have to learn his DMs). Which leads me to the next point...

How to tackle a new Battle LV 99 playthrough?
Now, this is a whole different thing, and you propably aren't sure whether this is a challenge for you. Well, I say: why not? You can always forfeit anytime. So, before you say anything, let me give you a few tips as I've done multiple Battle LV 99 playthroughs and came victorious:

• The most obvious yet important tip is that you don't have to play on Critical. This mod applies to every difficulty and, honestly, Beginner and Normal are still pretty easy even when everything is LV 99. For sanity's sake, you should begin with a New Game+ though.
• Never fear, Gummi stuff and Caribbean ship combat are not affected by these mod changes.
• You will level up extremely fast. You'll probably leave Olympus at the mid-fifties, so you'll already have every Level Up ability at your disposal by then (including the precious Second Chance and Withstand Combo).
• The toughest battle of all is probably the one in Olympus inside the collapsing building because it's hard as hell to kill everything before time runs out. Also because you still lack Airstep, Shotlocks and Links at that point. While it is possible to win, you'll probably get frustrated way before that, so use EZ codes to your liking.
• Hidden Potential actually scales extremely well for LV 99 enemies. Because yes, this game HAS scaling (this ability is the "turn KH2 scaling on-off button"). So, you will want to use Formchanges at all times (I actually find it really strategic that base Sora doesn't have Hidden Potential: he's not that helpless without it, but still using a Form will provide a very nice damage boost).
• Enemy Gigas are both deadly and HP tanks. So in Toy Box it is absolutely essential to ride in Gigas, use Countershield + Critical Counter or simply run away (because yeah, there's no shame in that). Don't worry though, Verum Rex stuff is unchanged.
• The HP amounts I've decided for bosses to have are in more cases than not lower than what they would normally have if they automatically scaled with Battle Level.
  

If you can get through Olympus, you can beat the game.

About compatibility
In order for this mod to work properly, you can't use other mods that change enemy attack data files for bosses (too many files to name; one or more per boss) or the Battle Level of the worlds (BattleLevelAuto_Table). Basically, nothing that changes HP. If it still doesn't work and you can't find the cause, rename my pak file with a few Zs at the beginning.

About installation
Either do it with the mod manager or install it manually. For the latter method, go to Program files/Epic Games/KH_3/KINGDOM HEARTS III/Content/Paks. If there are no more folders inside Paks, create a folder named ~mods (just like I wrote it). After creating it or if it's already there, simply place your pak file within your ~mods folder. That's where most of your mods should be installed. If you need further instructions, hit me up with a comment or a DM, or look for more info (it's everywhere) in other mods.

Credits
Mods used in the header pic:

• Ultima Weapon Plus
• KH3 Donald and Goofy Reimagined
• KH1 Sora colors replacement