Polyurethane finish

What are you planning on doing when you take the poly surface off?

It would be best to start fresh....remove the finish with Citristrip

It depends on what you are doing with them next. And if you are painting, it depends on what type of primer you plan on using and what type of paint.

Are you going to paint them or re- stain them?

just sand them and use a good quality paint it will be worth the expense

If you plan to just paint them.

Make sure they are clean and dry. Remove the doors and hardware. Mark the doors and cabinets with tape where they go. Lightly sand the doors and cabinets to remove any gloss and roughen the surface for paint with 120- to 220-grit sandpaper. Use a tack cloth or damp rag to remove dust after sanding. Prime with a stain blocking primer like Zinsser 123, KILZ,or BIN and have it tinted to the color of the top coat. This will prevent dark or stained surfaces from showing through the top coat. Acrylic, or water-base, paints are low-fume and clean up easily with water. Alkyd, or oil-base, paints require good ventilation because the paint contains solvents that can irritate your lungs and make you feel sick. Alkyd options require mineral spirits for cleanup, but they provide a hard, durable paint finish. Whichever you use, buy the best-quality paint you can afford for a lasting kitchen cabinet finish. Seal with at least three coats with a water based polyurethane. Use a small foam roller and foam brush for a smooth finish.

I did it once years ago to cabinets that had darkened over time to almost black from an already fairly dark stain. We took the cabinets down to do it just to not create a mess in the kitchen. We were so happy with the color that all we did was clean them up and poly them as is and didn't even stain them at all. Brightened the kitchen up incredibly! Stripping made it easier and less dusty mess to get down to the bare wood.

You will need to take the doors and hardware off. mark the doors and drawers so you will know exactly where they go as well as the hardware unless you are replacing that. Buy a stripper, I use Kutzit (works great) a little pricey though. After you have scraped all the old stain off down to the bare wood, you will need to give it a good wipe over with some denatured alcohol, then sand each door and drawer. Then wipe over well with a tack cloth, which will remove all the small bits of sand dust. Then you're ready to stain a lighter color. I would use the General Finishes gel, you can put it on with a rag or brush. Go to the General Finishes website and they have videos of exactly what you need to do. Good luck and post a pic when you're done.