I am about to put a set of KYB AGX adjustable dampers on my classic along with Eibach springs, is it wise to change top mounts as a precaution? At the moment, I am not experiencing any top mounts knocking and my mileage is 75,000 but will the oem top mounts last to say 125,000 without changing. At what mileage do top mounts start to fail.

 

I wouldn't worry unless you wanted adjustable one's, I've seen quite a few high mileage examples without any problems.