When metamorphism has only barely changed bedrock it is called low-grade metamorphism. When heat and pressure are so intense that they have dramatically affected the physical and chemical properties of crystals it is called high-grade metamorphism. If the rock is melted it is no longer metamorphic.
NOTE: When pressure and heat finally melt the rocks, the result is NOT metamorphic rocks, it is igneous rock. This is the big difference between metamorphic and igneous rock. Igneous rocks were once melted rock.