300Tdi and later have grease already. Land Rover removed the drain plug in their infinite wisdom (cost cutting and all that). Personally I prefer the older oil-filled swivels but that's just me!
You might be able to drain it out by removing one of the bottom swivel pin bolts. I'd take the weight of the vehicle off the swivel first on an axle stand, and remove the road wheel. You might have to take the car for a good run in order to get the grease warm enough to run out before doing this.
When drained just re-fill through the fill hole, easier to do this on full lock elsewise the CV is in the way!