Generally speaking, I feel that no one stands still in time: people change over the years. And because of that, if you're an artist, so will your work. I find this neither 'good' or 'bad', its just the way it is.
Having said that, and replying to the question, I feel (from the very limited set of artists I have enough knowledge of) that both Marc Silvestri and Jim Lee have become much better artists over the years. And for me it doesn't even necessarily have anything to do with 'style changes', I just feel that over the years they have become much more skilled at performing their art.