Hi,
A higher downswing plane usually results from your backswing 'looping' over the top of your transition.
That typically is a result of too much muscle tension buildup on your backswing causing you to initiate your downswing with a shoulder turn...the effect will be an outside-in club head swing path causing a slice/fade shot.
Try to focus on restricting your backswing slightly to reduce muscle tension build up in the backswing and then initiate your downswing with the hip rotation and let your shoulders lag behind...
To make the downswing even more effective, keep your wrists 'cocked' for as long as possible from the start of the downswing so that the hand release lags the shoulder turn....this will help you accelerate your downswing more by keeping your swing MOI as low as possible.
Good luck!