If it were me, now in hindsight, I'd get a new 5-wire to run. I spent hours trying to get a new thermostat working on an old 3 wire setup and gave up on that; was having too much issue trying to bridge pins and stuff. Once the new 5 wire cord arrived I taped it up and pulled it through, connected it to furnace board & thermostat and it was a 20 minute ordeal.