Create a Material Parameter Collection in the content browser and set a Scalar parameter inside it, then create a Collection Parameter node in the material, for roughness lets say, and pick the collection and parameter you created. Finally you can go to the level blueprint, create a Set Scalar Parameter Value node and execute it whenever you like by using a timeline to make the transition.
For this to work properly for your situation you may want to multiply the collection parameter with material instances’ scalar value though, instead of using a single value for the mat. instance.