Unfortunately, the only way to lighten the floor and keep all that beautiful wood grain visible is to strip it down and start again. Varnish stripper may or may not pick up wood sealer, so a light sanding is often required. Be conservative as wood floors can only sustain a limited number of sandings before the wood becomes too thin to withstand annual temperature and humidity fluctuations, general wear and the weight of furniture; ensure that your floor can sustain another sanding before renting the orbital sander again.
Three types of bleach lighten stripped or sanded floors. Use A-B bleach, a two-part compound containing sodium hydroxide and hydrogen peroxide to lighten sanded wood; it also bleaches some pigments in stains. Calcium chlorite, or chlorine bleach, lightens only stains containing dyes. Oxalic acid, used on tannin-rich woods like cherry, oak and mahogany, merely removes discoloration caused by rust and moisture and does not bleach wood or stain.
Soften dark wood’s appearance with cream or white glaze or tinted varnish applied to achieve a pickled finish or wiped on to achieve an antique finish. Use oil-based color on top of oil-based varnish; latex will not adhere. Sand the finish and clean the floor thoroughly to remove dust before beginning and add several drops of the original stain to the light glaze color so it blends well. Stay off the floor for two or three days longer than the drying time listed on the packaging so no solvent is trapped when you apply the finish coat with the same varnish used in the original finish.
Paint the entire floor, make patterns or stencil the floor using oil-based or water-based paint, depending on the type of varnish undercoat. Rough the finish up with sandpaper or with a buffer with a light stripping pad. Use painter’s tape to outline simple linear patterns, use cut or purchased stencils for repeating patterns or cover the entire floor. Use faux effects such as marble or paint a primitive rug design. Apply several thin coats and allow extra time to dry.
Purchase light-color area rugs to cover the dark floor. This will not change the floor itself, but occupants will perceive it as lighter. Look for rugs that match the style of the room and feature the color of the floor in their design. The rug will not only lighten the floor; the light rug and dark floor will provide dramatic contrast. If the floor is polished with paste wax or silicon-based polish, rugs may be your best option. You may even grow to appreciate the darker wood.