protected groups for media downloads with membership plugin.

I have set up the protected group so that the real path to the downloadable media is not shown, so all that is working,

Problem is I uploaded a large amount of media prior to adding the "path remapping" feature,

so my question is... How do I change the "protected group" of all that media from "none" to the group I want it in, without editing each media file individually?

Thanks
Trevor