I have a datapack, and in the data folder its just this:
data > alloy_forgery > alloy_forges
Within the alloy_forges folder I have a blank JSON file for each pre-existing forge (example: "bricks_forge.json"), except for 'deepslate_bricks_forge' where the tier was changed to 2.
None of the changes are applying- all still exist, and the deepslate brick one remains tier 1. I tried this initially through kubejs, but its also not working as a normal datapack.

Fabric 1.20.1, alloy forgery 2.1.2, owo 0.11.2

Due to the nature of when datapacks are loaded they are not supported directly for registering forges. They are generally world-specific (looking away frol systems like KubeJS), and the data needs to exist before registering the controller blocks.
You need to use the moddata folder instead. This is explained on the wiki: https://docs.wispforest.io/alloy-forgery/defining-a-forge/