Begin with a screening
Flooring wholesalers and veteran floor finishers agree that you can recoat water-based poly with oil-based as long as the original coating has completely cured—generally 30 to 60 days. You'll have to lightly buff the old finish with a power buffer to prepare it for the new polyurethane.
Rent a power buffer and buy a fine (150-grit) screen. Use a light touch to avoid cutting through the finish and into the stained wood. Before applying the polyurethane, vacuum the floor thoroughly and then wipe up the remaining dust with a cloth dampened with mineral spirits.
Ventilate the area well and wear a respirator rated for organic fumes (follow the label warnings on the polyurethane can) when applying the finish. Stay off it overnight and give it a few days to cure before bringing in furniture or laying rugs down (or they may stick permanently!).