That elbow is almost certainly lead, not cast iron. If you take a nail, or something similar, it should scratch easily and be shiney underneath that surface corrosion. It looks in good shape. The cleanest way to lower it might be to pay a plumber to do it. He could sweat a new flange on it after you get the new floor in. The proper place for the flange is on top of the FINISHED floor, and anchored down through the subflooring. Depending on your skill level, you may be able to do this. I think it would be less work than trying to remove the elbow.
If you want to replace it all with pvc, then you have to pull it all out of the hub in the CI, or cut the cast further back where it is the nominal size so you could use a no-hub connector.
There's likely a brass ferrule in the hub that the lead is sweated to that you'd have to remove to use a donut and pvc if you want to remove it from the hub.