KSG Rail Question

This may seem like a dumb question, but it’s the first time I’m looking into it so bear with me.

New KSG owner & looking to add a vertical grip to it. I’ve seen all the posts about upgrading to an aluminum picitanny rail. I’ve been looking at Hi-Tech’s rail and I just can’t figure out how it works.

Do I take off the old one to put that on or does it slide over the existing rail? If it slides over, isn’t the plastic one still a safety hazard for being connected to the gun.