Recoating a water-based acrylic floor finish with oil-based polyurethane is not usually a problem, so long as the old finish is properly buffed and cleaned before applying the new finish.
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!).
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
You'll also need a power buffer, sanding screens, and a respirator rated for organic fumes.
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.