You have a few different options. First, clean and sand the cabinets with 220-grit, or 320-grit, to reduce the gloss. If the existing paint shows no signs of stains or tannin bleed, you can prime them with a quality latex bonding primer. Sherwin Williams sells a latex product called Extreme Bond primer. Note: Latex primer won't prevent bleed through. I only recommend it if the cabinets are in good condition. That bonding primer I recommended is also good if the existing paint is oil-based. It will provide a solid bond to prevent cracking and peeling with the new paint. If the existing paint shows signs of bleed through, use an oil-based primer (Cover Stain) or shellac-based (BIN). If the cabinets are peeling, or badly chipped, strip them, clean, sand, prime with either product I mentioned, and paint. When you paint your cabinets white, you might see things that you don't see now because they're green, so if you want to be totally safe, you can just prime them with oil-based primer to eliminate any chances of bleed through with your new paint.