Sorry Andy, but I think you've out-smarted yourself on this one. IF bushing design isn't allowed to be changed, that means you can't replace a compliant stock bushing w/ one that is not compliant. In other words, you can't replace the stock rubber one, that's all squooshy, w/ a metal one that allows for no out-of-axis motion. THAT is a design change. If you're going to use the 'bushing material is free' arguement, the counter to that is 'no allowed modification shall perform a prohibited function'. So, while you can use a metal bushing, if it changes the design from a compliant to a non-compliant configuration, that's a prohibited function.