I am thinking about adding insulation and subfloor on the basement bathroom floor like other rooms, which will rise the floor level about 2". Do I have to break concrete floor to redo the riser? Can I use the attached flange to avoid modifying riser?
It's hard to tell from the picture, but it looks like you have a 4" cast iron hub in the floor. If it is a hub that flange will not fit. It's made to go into 4" pipe it will be to small to fit into a 4" hub.
Just be sure you get the inside of the pipe clean. And be sure to screw the flange down to the floor. There is another flange that I prefer. It is the same as the one you have but it has a stainless steel ring.
I'm assuming that you got 4" cast iron stub-out currently flush with concrete slab. Measure distance of top of the rubber gasket to bottom of your toilet flange. if you have 2" there you will be fine. After you finish your new floor, bolt your flange TIGHT to finished subfloor then tighten small allen screws on the inside of toilet flange to expand rubber gasket and seal connection.