There are 3 macronutrients

Carbs
Fat
Protein

If you decrease one percentage, another one has to go up, or both of the others, as it will always equal 100 % of your calories

You can do 33% of each

33.3% calories from carbs
33.3% calories from fat
33.3% calories from protein

If you decrease carbs, the percentages have to go elsewhere:
5% calories from carbs
60% calories from fat
35% calories from protein

You need a certain amount of protein to maintain your muscles and repair needs for your body. Less than that will be muscle wasting, and more than that the body converts excess protein to glucose.

So protein is your constant, it must stay about the same.
So if you decrease your carbs then fat HAS to go up.

You can eat more, or less, but the PERCENTAGES always equal 100%