This will be a little tricky but doable. Don't use the strongest paint/varnish remover you can find, at least not at first. Also experiment on a hidden area. You will need to know the dwell time so only the polyurethane is removed and use fine or ultra fine steel wool.
One possibility is to use a strong solvent to loosen the finish. Chances are 50/50 this will work and it could make more of mess but if you have some lacquer thinner or xylene handy give it a try. You will still need some steel wool for this method.