Just based on what I know (and others hopefully will chime in on this who have far more knowledge). The big thing is isolating the wheels from each other, then rewiring electrical pickup to be on one side while the other remains the negative return, might also involve internal wiring depending on how they bring negative to the engine)
In terms of legacy working, it should work fine with 2 rail if the conversion works. The actual signal is sent through the house wiring, so that doesn't change, as long as the track wire is on the negative rail and is available in all areas of the layout, should work from what I know. I suspect the devil is in the details with this, trying to isolate the wheels is likely to be a bear, especially the drivers.