So what will happen if it is added to an existing save....will it break the save or the save wont load at all?

It should not break the save.

The mod does not add any new buildings, resources, recipes, prototypes, or saved data. It only changes existing values while the mod is active.

So if you add it to an existing save, the save should load normally and the affected values should update after loading.

If you later remove the mod, the save should still load, and the game should return to the default vanilla values.

That said, I still recommend making a backup save before testing any balance mod on an important world, just to be safe.