Fred,
I doubt it is bolted from below. You should be able to remove the bolts, normally lag bolts going into the wood of the roof, once the sealant (probably Dicor) is removed. The hole will only be large enough to route the cables through. After removing the dish, reinstall the mounting bolts and screw a piece of aluminum, etc. down to cover the cable access hole. Make sure you can access the cables in ase you want to add another dish later. Then coat the bolts and plate with Dicor to seal them.
Steve
Steve