So I just figured out how to mod worlds on legacy Minecraft on the Xbox One, (Minecraft Xbox One Edition) and I decided to make a guide of how to do it yourself! Without having to convert to bedrock format!

Requirements:
An Xbox 360 Console
An Xbox One Console
An Xbox Live Account
A USB Flash Drive (8GB Minimum)
A Windows PC
Internet Connection
A Copy Of Minecraft Xbox 360 Edition
A Copy Of Minecraft Xbox One Edition
Horizon Modding Tool
Universal Minecraft Editor (Original Link is dead, use this link instead.)
The Provided Minecraft Xbox 360 Edition Title Update File, Or Any Other Title Update That Came Before Update Aquatic.

Let's Begin!

STEP 1: Downgrading Minecraft Xbox 360 Edition
Download the required title update file of the game HERE, Or any other title update that came before update aquatic.
Plug in a USB flash drive into your Xbox 360 Console. On the dashboard, go to "Settings" and select "System" then "Storage".
Format the USB flash drive if needed. Find "Minecraft Title Update" On your Internal Drive, then move it to the USB flash drive.
Eject the USB flash drive and connect it to your Windows PC. Open Horizon and open your USB flash drive.
Under "Title Updates" select "Minecraft Title Update" and select "Extract Selected File" and select a location to save the update on your computer for backup. Then delete the title update from your USB flash drive. Click "Inject File" and navigate to your downloaded title update file and select it, wait for the process to complete. Eject your USB flash drive and plug it back into your Xbox 360 Console. Make sure "Auto Downloads" is disabled, go to "Settings" then "System" then "Console Settings" and disable "Auto Downloads", this will prevent Minecraft from automatically updating.

STEP 2: World Setup
Launch Minecraft Xbox 360 Edition, when the "Update Required" windows appears, press "B" or "Cancel". Just in case, configure your settings to your liking, sometimes downgrading this game can mess up the settings. Click "Play Game", then select "Continue Playing Offline" on the windows that appears. Select your USB flash drive on the Device Selection window. Create a new world that you can earn Achievements on. Once you loaded your world, save and exit the game. Then close Minecraft. Eject your USB flash drive and plug it in to your computer again. To backup your world, open Horizon and under "Games" and "Minecraft Xbox 360 Edition" find your world and extract it to your pc.

STEP 3: Modding Your World
Open Universal Minecraft Editor and select "Xbox 360" then select your world. Now you can mod your world, like toggle permanent Invincibility, to editing your inventory. Once you have done all the modding, click save, then close Universal Minecraft Editor. Eject the USB flash drive and plug it back into your Xbox 360 Console. Open Minecraft and load your world and test your mods, then close the game.

STEP 4: Updating The Game And Uploading Your World
Eject your USB flash drive and plug it into your computer. Open Horizon, delete "Minecraft Title Update" and inject the latest title update file you backed up, or you can download the latest update from your Xbox. Plug the USB flash drive back into the Xbox and load Minecraft. Click "Play Game" and select your USB flash drive. Once you find your world, DO NOT LOAD THE WORLD OR IT WILL OPTIMIZE IT FOR UPDATE AQUATIC, THEN YOU WON'T BE ABLE TO PLAY THE WORLD ON LEGACY XBOX ONE EDITION! Press Open the "Save Options" menu and select "Upload Save For Xbox One" and let the world upload to the cloud. Once it's done, Turn off your Xbox 360 and switch over to your Xbox One.

STEP 5: Downloading Your World Onto Your Xbox One
Launch "Minecraft Xbox One Edition". Press "Play Game" and Press "X" to download your modded Xbox 360 world. Once it's done, load up the world and test your mods, then that's it!!! Enjoy your modded Minecraft Xbox One Legacy Edition World!

 

[CompSciOrBust]

I used to love the MC360 save modding scene back when I was in high school. Was basically the only fun thing I could do without a JTAG.

Here's the universal MC tool for anyone that needs it since the link isn't in the OP: https://www.universalminecrafteditor.com/

 

[Visualwit]

I used to love the MC360 save modding scene back when I was in high school. Was basically the only fun thing I could do without a JTAG.

Here's the universal MC tool for anyone that needs it since the link isn't in the OP: https://www.universalminecrafteditor.com/

The universal minecraft editor was replaced by a new paid tool called universal minecraft tool. I have uploaded the old installer to my Google Drive and added it's link to the OP.