If an app wanted to be able to switch something from public to private or back, it'd get a token with write permissions for two names, one with the prv.* prefix, and one with the pub.* prefix. And that's a good thing: it makes it clear to the user that the app is requesting the ability to make the things that they write public.
It's still just one grant request, it's just that there'd be two line items instead of one. Also, names are simple pointers to blocks of data, so, for instance, if both names happen to be pointing to the same written piece, there's no duplication of data.
If an app wanted to be able to switch something from public to private or back, it'd get a token with write permissions for two names, one with the prv.* prefix, and one with the pub.* prefix. And that's a good thing: it makes it clear to the user that the app is requesting the ability to make the things that they write public.
It's still just one grant request, it's just that there'd be two line items instead of one. Also, names are simple pointers to blocks of data, so, for instance, if both names happen to be pointing to the same written piece, there's no duplication of data.