I really like the style Oda has been using lately. Especially with Chopper, at first he looked really bizarre. But now… it should be illegal to be so cute. Is it just me or has he also been toning down on muscles? Something about Zoro is different since W7 started at least. Maybe he's just drawing necks different or something...
Art style with a mangaka is always going to change, for long series like this especially. Some series you can barely recognize the art from the first volume from the current stuff (Ippo is a really good example of this).