I similarly struggled with changing my downswing path from steep/over the top to a flatter in to out path. Despite knowing what I had to do from detailed instruction from a coach and video taping each day what I thought was the needed swing adjustment, there was insignificant change either due to habit and/or muscle memory.
The key trigger I came upon almost as a last resort was starting to do a number of "super-loose" practice swings not stopping between each swing - so just back/forward and back/forward with loose grip, loose wrists, loose arms, etc etc and on those swings ensuring I was starting the downswing from the lower half of my body. As I was just continually swinging back & forward, I found it easy to even further loosen my swings and also found it was quite easy to over-exaggerate the key elements I needed- hip bump, right elbow in, in-out path. I video-taped these practice swings to confirm that finally I was achieving a totally different downswing path that was flatter and in-out whilst still ensuring my backswing path was going back squarely.
I then took this too the range to do 3-4 practice swings before hitting a practice shot and then to the course where I did the same loose swings before my shots. Whilst there are still elements I need to work on, the above process allowed me to make a significant change after all previous efforts totally failed.
Like I guess all "triggers" what worked for me may not work for everyone, but strongly suggest you try it, if like me you felt you had tried everything else to no avail.
