Charging for premium features - Sure. But the basic version will remain free according to the Trello Guide.
Free... for how long?
Trello is free forever. We may add pay-only features in the future, but everything that's free today will be free tomorrow and forever. Also, we'll not make a for-pay feature that forces you to compromise on privacy, security, or portability. Those are built-in. [...] Trello is free.