Connie....the far majority of wood stains are normally applied to bare wood then a protective top layer is applied...In commercial factories this is often a Catalyzed lacquer, not a poly. If you are looking to darken them you could apply a new layer of Polyshades (stain and poly together) but I have found that product to be a real challenge to make it look good. As with any re-finishing project good solid prep work will prpvide the best results and in the case of kitchens a deep cleaning is needed to remove any hint os of soiling or oil / grease residue. TSP is often used as s final step.one addtional issue with "sanding" is it provides a bit of "bite" for the new finish.
STAIN GEL AT HOME DEPOT BUT SAND FIRST................JIMMYG WPB FL.
There's a product out now that requires no sanding - I've refinished two items now with it and it looks great. It's called Rust-Oleum Cabinet Transformations and it comes in many colors. You can find it at Home Depot. :)