I have two offset cast iron closet or toilet flanges on 4" cast iron pipe that are set in concrete. The flanges are above the level of the concrete. The flange brackets are rusted and the wax ring doesn't fit properly. Is there some way I can replace the cast iron flange with a PVC one that will attach to the 4" cast iron pipe?

They do make one that goes inside the cast iron and it has stainless screws to tighten that expands a rubber gasket that seals against the cast iron, not sure if they make that in an offset. Could you just lead a new flange in place?

How much offset do you need? There's a 1" offset compression flange available.

There is no good, or proper, way to attach a plastic flange to a cast iron riser. i assume you have the oval flanges which means you need part of a second wax ring fused to the first one to make a seal that fits it. A "rusted" cast iron flange is usually still usable unless it has broken.

if you really wanted to transition to PVC you would have to cut the concrete, break up the concrete, dig down to expose the riser, cut the pipe, use appropriate PVC/CI shielded coupling and then YES transition to PVC. BUT the advice given already is great, except that offset flanges by their nature have a pt. of constriction which could be future problem with clogs.

IT depends on the offset's design. Some have a smooth sloping transition, but they also requires more space between the top of the riser and the finished floor. The shortest ones have a "shelf" under the toilet opening, and those will cause problems.

another option is to install a Sioux chief Ringer...its just a SS wc ring...it can go over your broken flange and screw into the concrete...the wax ring will squish into the ring and seal it...ive done this a few times when there's no good option...it works

IT will NOT work, if he has the "oval offset flange whick would be likely for a cast iron system.

Use wheel grinder to lop off the extra iron above grade. Add stainless steel or brass closet ring and set pins into concrete to hold down flange. Use appropriate wax to fill void and insure water tight connection. Not elegant, but serviceable and no jackhammer work.