HLJ there is a Kit you can get at Lowes for that there are 1/8" 1/4" and i think like 3/8" spacers , gaskets and all
you line up the spacer with the old ring , drive a screw Thru the Old pvc ring raiseing the new ring up to floor height ,
then install as per normal
https://set-rite.3dcartstores.com/
http://www.ehow.com/how_6107135_inst...er-flange.html