How to change the mining level of already existing ores and tools in Minecraft?

[EmeraldWorldLP]

I am currently creating a mod for 1.12.2 and I can't figure out how to change the Mining Level of Vanilla Tools and Blocks. My mod changes the balance and progression of the game. I can't find anything about this on the Internet and I know that there are mods which do this (BetterBeginnings). Any Help would be appreciated!

 

[Celestialphoenix]

https://minecraft.curseforge.com/projects/material-changer
Edit, sadly this doesn't cover blocks- but it'll sort you for tools, armour ect...

ContentTweaker lets you add your own blocks and items to the game, so you could re-add the blocks you wish to tweak, and pull a switch-a-roo with CraftTweaker
https://minecraft.curseforge.com/projects/contenttweaker
https://minecraft.curseforge.com/projects/crafttweaker

Edit (again!).
Accidentally misread your post as 'modpack' and not 'mod'. Sorry mate.
(I guess if those mods are open source, you could take a look and see how they did it)