While having my R53 serviced for something unrelated I was told that the passenger rear lower control arm had been replaced. I was told that it was a different revision than the upper and should have been replaced as a pair. I believe that this was done as part of a collision repair by a prior owner. My question is why do they need to be replaced as a pair? They are OE fixed length bars, nothing fancy. The only possible wear item would be bushings. If they weren't compatible the car couldn't have been correctly aligned. Do I need to spend close to $500 including alignment to replace one upper? And, what about the other side? Are two old and two new OK?