Had the plumbing roughed in when house was built and finally thinking about finishing basement. The shower drain is cast and below grade. I need to move it about 2 feet over. Looks like the have a donut fitting going from main4 inches to 3 inch bell. Ptrap is under slab since it holds water. Can I remove the cast fitting and put fernco donut in to change to pvc? If so do they sell 4 to 2 pvc reducer? Do I need to move ptrap to be directly under shower drain? Let me know your thoughts I have attached 2 pics. The drain and the other is the vanity drain which the plumber leaded the pvc to cast.