As said before, check under the trailer to make sure there isn’t already a retainer plate fitted in a shorter position. If there isn’t, you would have to pull the floor up over the rubbing plate area and see where the crossmembers are. It is perfectly feasible to cut a hole in the rubbing plate, weld in a new retainer plate then bolt a new kingpin into place but you would have to know what you were doing and would probably have to weld in additional crossmembers. Bear in mind also that if it’s a 2006 trailer it will have some amount of wear on the rubbing plate already, so it may be worthwhile replacing the rubbing plate complete and fit the kingpin in the desired place. As stated before if you change the position of the kingpin you may have to submit a notifiable alteration. To replace a rubbing plate budget in the region of £2000 to £2500.