Good point. The interviewees had an understanding that this content will be freely available and now that's changed. Then again, maybe he told them the content will be free but there is a possibility that one day it will not be free anymore. Either way, the better approach would have been to leave existing interviews freely available and start applying the pricing structure from the present onward.
On another note, I'm glad to see Andrew turning Mixergy into a real business that can be self-sustaining. As entrepreneurs, we should all be able to agree with that.