When it's time to 'upgrade', what constitues one?
Outside of the context of hi-fi, an upgrade is a general change for improvement purposes. Therefore, if you wanted to upgrade your car, you would get something better than your existing model.
Given that just about every bit of hi-fi kit colours the sound in some way, what else could constitue a quantifiable upgrade other than perhaps improved build quality or less distortion, for example?
Perhaps 'upgrade' would be better phrased as 'change'?