you just need to set the post process blend weight. you can modify its value in the details panel with the camera selected (camera settings section, see picture) and you can modify it via blueprint by using either of the nodes shown in the picture below. a value of 1 will use the players camera post process and a value of 0 will use the world post process. note though that some effects cant be blended but are more like a on to off transition. i tried using this with a post process material asset but it could only be on or off no in between.
