Except that their terms of service (at least now) say exactly the opposite:
> In the event your paid Prezi account reverts to a free Prezi Public account, any content you previously created with a paid Prezi account and designated as Private User Content will remain Private User Content, but you will not be able to edit such content. New content you create with such an account will become Public User Content, which means any new content you create from that point forward is going to be public.
> In the event your paid Prezi account reverts to a free Prezi Public account, any content you previously created with a paid Prezi account and designated as Private User Content will remain Private User Content, but you will not be able to edit such content. New content you create with such an account will become Public User Content, which means any new content you create from that point forward is going to be public.