A couple of thoughts. In some cases the change in style is not so much a change in style as becoming comfortable doing your own thing and not being bound to the 'house style'. Without looking up interviews I'd say Sienkiewicz and Mignola fall into this category.
Second, because comic books are stories and not just pieces of art, looking back, I recognize that I can't appreciate style completely separate and apart from storytelling. The ability of 'cool' without any substance to carry a 22 page book is, at the end of the day, limited, in particular in retrospect. For this reason, I have a problem with artists that I feel abondoned their own voice in favor of the flavor of the time, even though I understand an artist wanting to keep fresh and maintain their own interest and being influenced by your contemporaries. If we take for example the Image style, a very large part of it confused filling every space with poses, lines and cross-hatching for actual storytelling. So while I was on board Cyber Force Silvestri, which at the time felt like Silvestri stepping up his game and showing he can be just as good as Jim Lee, now none of his work has the same appeal me as the older, cleaner work (certainly when you look at the original art that in many cases looks better than the printed, colored page).