Depending on the speakers it may not be a downgrade. He is moving from factory speakers to a high quality coaxial set, which will completely blow the factories away.
Coaxial speakers do have some benefits, one is that high- and low-frequency sounds originate from the same point in space. This "point source" delivers the full range of sound frequencies in correct phase alignment, resulting in a smoother frequency response and improved clarity. This is a good thing considering most people who install a component set will mount the tweeters in generally poor locations, not taking into account speaker phasing and imaging.