I have a bathroom in a garage apartment which floods every time we get rain. :( I am in the process of installing drainage lines, but the elevation difference between the back of the yard and the front is marginal. The way it rains around here, flooding is always a risk. To mitigate the risk, I am pouring 5" of concrete down on the floor of the apartment and the bathroom contained within.
My challenge is the toilet flange. The house is 70 years old and the flange it solderd on with lead. Is there a flange that bolts onto the existing flange and extends it up ward where a new one can be installed after the concrete is set?

unless you open the floor and change the pipe you may end up with a rig job. Guessing... is the riser a 4" lead riser?
try a 3" pvc couple glued to a 3" pipe with cardboard wrapped around it so you can install a 4" x 3" flange on the outside of the 3" pipe resting on the finished floor. Use silicone caulk to seal the couple to the lead. The 5" concrete with seal it the rest of the way. Make absolutely sure the riser stays perfectly straight up.