Staining lighter to darker cabinets

[B]Staining lighter to darker cabinets[/B]

Newbie here (both in forums and in home ownership)..thanks in advance for any help and patience!

So we just bought a house and I have "some" DIY skills and decided to jump head first into refinishing (sanding and restaining) the kitchen cabinets etc. We are going from a very light stain to a very dark stain.

My question is pertaining to how much I need to sand...I am willing to put in the work, but don't want to waste my time either if it's not needed. Basically I have sanded the doors and gotten much of the gloss finish off, but the light stain (kind of a white-wash color) is very stubborn to sand out. The problem is that when I have test-applied some stain, you can tell the difference in the areas where I was able to get down to the bare wood and the areas we couldn't get the stain off. It's not real glossy but you can definitely see a difference and it does have kind of a satin sheen where the old stain wasn't off...and the color looks the same with the obvious difference in the way it reflects. But.. that is before applying polyurethane.

I have also tried a chemical stripper to the doors, but it didn't really remove the stain either.

I am applying a matte poly over the stain afterward. I have already done the cabinet boxes and they look great. But, those doors are kicking my butt. So...assuming I get the poly (or whatever is sealing it) sanded off...will the new poly smooth everything out (as far as the gloss/matte) once I apply it at the end?? Or do I need to just get to the grind and put the work in to get everything down to the bare wood?

Thanks again in advance for any advice!