Just to make sure we're talking on the same sheet...a rough-in is measured from the finished wall to the center of the flange. If you flange does not have a set of slots for bolts at that center point, then you may have to replace the flange or maybe use a repair ring if you can cut a section out for there to be room to install the bolts. This is not something I've got direct experience with. My guess is that the flange is CI on a CI pipe. Depending on if you can access from below, you might just rent a snap cutter, cut it off at a convenient point, then replace with say pvc, then, you could move it to anywhere you want and make it a 12" rough-in as well. You can use a 12" rough toilet on a 14" one, but you'll end up with two more inches behind it.