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. If I need to lower the grade, I also keep this to only one side of the tree.
Trees get roots cut during construction operations all the time.
Cutting some roots on one side of a tree does not make the tree happy, but if the balance of the root system is undisturbed and the soil on top of it is not compacted by heavy equipment or foot traffic, a healthy tree will grow more roots and continue on its way. Obviously, when you need to do major work around trees, you have got to keep your wits about you and think about the consequences of your activity before you act. In fact, they will think they are in heaven and send feeder roots up into this mix as soon as they can. Your plants will, however, have time to get established before the tree roots start to compete for water and nutrients. The mix I use depends on what I want to plant and what I have on hand.