Stain or paint???

If you are going with a staining, you will need to strip off the old finish. Using a chemical stripper is the best way to go. Problem with sanding, besides being labor & material intensive, the sander does not know where to stop; the stripper does. Yes, stripping is a mess, but it’s soon over and does a better job of getting out the old stuff.

The best way to determine how the color is going to come out is to run a test panel, using your entire finishing process. Getting half-way into the project and realizing the color is not where you want to be is too late.

Lastly, are you positive that the finish is polyurethane? This is very rarely used by factory or custom built cabinet shops.