You will not be able to re-stain your sealed cabinets. Once stained and sealed they would have to be sanded to raw wood which would take forever and never look right. I would advise painting them.

What color? Darker or light color in granite?

I would paint cabs white (you could paint dark brown but I think it wold make the kitchen look quite dark and oppressive) or have the cabinets resurfaced in whatever wood or colour you'd want to match the granite. I'd go with nickel/ chrome hardware...just my two cents....

you could refinish them to a new stain color but it is harder to do than painting them.refinishing them is double the cost of painting them

You can over stain the cabinets if you want them darker. Search for how to on Pinterest or google it. Just clean very well! Rinse and rinse again. Sand lightly apply glaze or stain to darken. Apply 2-3 coats poly.

Be carful with applying stain over the cabs like they now they do sell products that you can apply over old finishes but traditional stains will not adhere to old coatings . Gel stains will also not adhere with out removing the the clear coat

If you're really interested in stain, do a few test pieces to see how well it works. Every piece of old wood is going to be different. If it's not going well, then you can easily switch to paint.

If you paint, I would definitely spackle in the little routed detail on all the doors to update the look.

I would definitely paint the cabinets. I don't mind the existing hardware. I would test the paint on one door first to see if you might like it.