Refinishing kitchen cabinets?

You have to strip the sealer off. Try citristrip,then make sure they are clean and dry prior to re-staining

You will need to remove every trace of that sealant. Any slight bit will keep stain from penetrating. I'd take another look at paint where all you need is sanding and cleaning.

I agree with Ken. You will need to sand all the sealant off and get down to raw wood. Since the sealant does get absorbed into the wood regular stain may not penetrate properly and you could get a blotchy finish. I recommend using a gel stain like Minwax Polyshades.

Gel stain Cabinet kit. It's a 4 step process but no sanding.

If you want to use traditional wood stain, you'll have to sand ALL the clear coat off. Very time consuming but will give great results. Be sure to get it all off, especially if you are using a dark stain.