Classic pieces are already added to the game. Only WCO participants are able to switch between pieces which you can change in the settings menu. Do note that signing up for the WCO will not instantly give you the classic pieces since they are added manually to each account.

I do wonder how will the world championship will work out in the play.stratego.com site. That site has so far very limited options. i hope it will be easy to handle it. Also i cant see how can i use the modern or classic pieces in the board. Is it something you are trying to implement soon?

Welcome