I don't think you'll be able to change the studs without removing the drum and the brake shoes at least. I had a look in the SIII parts catalogue and by the diagram it looks like the wheelstuds have splines on them and are pressed into the hub from the inside. You might be able to knock them out with a hammer with the hub still on the vehicle but it might be a bit tricky knocking the new ones back in without taking the hub off. I'd just take the hub off because it won't do the bearings any good if you go hammering very hard on the old wheelstuds.