Nice. But that is lie. They can upgrade Pleione.
Because I saw exactly same situation - Ragnarok Online - it have even older engine. And I saw two fan worked fan projects that completly replaces Ragnarok Online own engine. One make game work in browsers via WebGL and one make game work on android tablets. And that done by small group (or even by one man) without millions from publishers.
I played game via that client on some private server and it's completly fine.
It's not a "process would be equivalent of writing a new game from scratch" - it's just devcat abandon game and milking their playerbase via new payd content.