I believe the lower leg bolts on the post-2009 18.5 WSM's were relocated due to issues with the vent tab locations.
Something tells me that no, the legs from the newer 18.5 WSM's will not fit the earlier ones.
A call to Weber CS should be able to confirm or deny this.
Also, you maybe able to buy a charcoal bowl from a 2009 and new WSM and use the middle section from your earlier model. Not sure of the cost though, might ask the Weber CS if you decide to call them.
Also, the legs may still be under warranty, so a call to Weber CS should be in order.