Leisure Travel Vans began the switch-over in March. Several have shown up at dealers already. Vogtrv had one in stock a couple days ago, last time I checked. Lamesa rv had one, but it sold already.
Probably depends on the Class C manufacturer, there are still a lot of older Sprinter's in some manufacturer's inventory to be converted. Your best bet is to make contact with dealers who will be carrying the brand of Class C that you are interested in and see if they can give you a possible date.