If you raise the grade too much all around a tree, you will cut off the air and water that those roots need to survive and your tree will eventually die. The same result is obtained if you lower the grade substantially around a tree - particularly a mature tree. Mature trees are not as adaptable to changes in their environment, sorta like mature humans. The older we get, the more set in our ways we become. Some trees are more sensitive to changes in grade than others, which is why you need to know your tree and learn a bit about it.
Many experts advocate no changes in grade at all around mature trees.
Again, simply, you don't. Whatever you add will change the grade somewhat. You also have to use some common sense. I prefer to make my changes to one, or at most two sides of a tree instead of encircling it completely. This leaves the bulk of the root system as it was.