I believe they confirmed, on twitter or somewhere I frustratingly can't find, that downgrades are charged at the correct opus rate, after a user asked and was told "either that's how it works or it's a bug"

With the subscription, it costs less to use opus in that it doesn't chew up our session however the cost/benefit is balanced against not performing as well on certain tasks. So it's not a straight up yes/no.

I don't think they charge if you downgrade, but if you upgrade from opus to fable they will charge you.