We recently tore out our old fiberglass tub/shower and replaced with just a walk in shower. I had the exact same issue, I had 1.5” copper pipe for the drain pipe, which emptied into the cast iron 4” toilet drain. Problem was, that cast iron right below the toilet is old, terribly rusted, and I didn’t want f to accidentally break it by trying to get the 1.5” copper fitting out of there. So I just left the 1.5” copper on the toilet side, and connected the new 2” PVC to it using a 3001-215 Fernco. I had to order it on Amazon, no local stores had it in stock.
If it’s just about reducing 2” to 1.5” PVC that should be fine, but do it far enough down the drain pipe where that reduction won’t cause a problem.
Here’s a pic showing where I did our reduction, all the way to the right you can see the fernco and the start of the 1.5” copper pipe. From there its only a few inches of copper pipe, then there’s a couple bends and it empties into the cast iron sewer pipe. The next two pics show the original drain piping, and in the last pic you can see the bends and connection to the drain pipe that aren’t visible in the first pic.
When we remodel the rest of the bathroom, we are planning to replace as much of that cast iron pipe with PVC as we can. My plan now is to make it PVC from the toilet flange all the way to where the shower drain connects. At that time, I’ll extend the new 2” PVC shower drain all the way. But for now the Fernco is working great and I haven’t had 1 single drip from it. Granted, I did use an impact drill/driver to tighten the bands, specs say to tighten to 60ft lbs of torque.
