Opera isn't switching to WebKit, it's switching to the Chromium codebase (which uses WebKit)
All the articles I've read recently, including this article, all only mention WebKit. Where have you heard that they are switching to Chromium and not just WebKit? I'd be interested to read more about that.
I can't find anything that suggests that they are using anything other that WebKit as the rendering engine. ExtremeTech has an article that mentions Chromium:
By switching to WebKit/Chromium, Opera will not only become faster and more functional, but it will also allow its Norwegian developers to focus on the browser’s overall user experience.
Notice that they seem to equate WebKit with Chromium, which isn't the case.
In my opinion (no based in knowledge of any kind) is that they are just ripping out Presto and replacing it with WebKit. I suspect that the next version of Opera will visually look similar to Opera now and will support most if not all the same functions (customizable menus, ad blocking, Mail client, etc).