I'm trying to replace bathroom floor tile and toilet. after old toilet removed. I found the flange is not level. the front side is higher than back (wall) side about 1/4". The problem is the flange is cast iron and soldered by tin on the metal pipe. Hard to replace and no any screw/hole on top (only two notches for toilet holding screws). the flange is not touch the concrete floor only holding by 4" metal pipe under.
a. Do not change anything, the flange was designed that way (?). just clean and put a new wax ring on it to install the new toilet.
b. use construction epoxy resin to fill the gap between the flange and floor to add extra support. let wax ring to handle the 1/4" uneven level gap
c. use a steel flange repair ring has extra screw holes outside of the flange area (because the old flange has no screw hole on it, so the standard screw holes on any extender/rings cannot go through). screw and force the front of the flange going down 1/4". Make it back to level evenly. (will this way broken the pipe?)
d. use something like Fernco wax free seal. don't touch the old flange.