Hopefully, the pipe in the slab is cast iron and not galvanized! There is lead between the CI pipe and the flange. To make that kind of a connection, they first pack oakum (oiled hemp) into the joint, then pour liquid lead, then tamp it all down. If the existing flange is not broken, you may be better off just cleaning it up and installing the toilet. If it is broken, and you are planning to add to the floor (say put in some new tile), then instead of replacing it with plastic, you probably should pay a plumber to install a new CI flange on top of the new floor. To do that, you'd need to leave some space so he could work on it. To change to plastic, you'd need to break up the floor to convert part of the pipe to plastic as well. Plastic joints are solvent welded (although they call the stuff glue). There isn't a good way to connect a plastic flange to cast iron, but there are ways approved to convert the pipe from one type to the other.
An alternative would be to use a new flange that has an expanding seal that fits inside the old pipe (if the pipe ID is 4"). This requires the inside of the pipe to be fairly clean and smooth, which doesn't look like the case on yours.
By far, the best alternative is to take the old one off, and install a new one, or, just leave what you have.
To take the old one off, there are a couple of ways. If you hit the ring you can crack it, then pry it out, then dig out the lead. It looks pretty tight into the concrete, so getting it out could be a pain. The other way is to make swiss cheese out of the lead with a drill, pry that out, then pull off the ring. There's a risk of damaging the pipe if you try to break the flange, but is probably the way a plumber would do it. Experience here helps.
