Should you replace good parts? At first glance, the answer seems to be, “No. It’s a waste of money.”
In automotive repair, it’s standard practice to replace certain “good” parts to save the consumer money. Consider a small bypass hose must be removed anyhow while doing work on the cooling system. Why put the old hose back on? It might look OK, but it’s had time in service and replacing it now is far cheaper than having it fail later.
A similar dynamic is at work in lighting maintenance. Relamping an entire building at one time, with all the tools and equipment ready and the downtime scheduled, costs much less than piece-meal replacing individual lamps as they go dim or fail.
Using the automotive repair example, suppose that during a PM you find a dead vibration sensor on a motor that has two. Replacing two takes only minutes longer than replacing one (job setup is the major time component). These were installed at the same time three years ago, and the failure is probably due to a dead battery. There is only one common-sense choice here.