If you delete a mirror's submirrors and reuse the slices that were part of the deleted submirrors, all of that data is gone from the system. Thus, make sure you back up the data on the mirror before deleting it and its submirrors.
When removing a mirror, you first "break apart" the mirror into its submirrors. You can choose to keep the data on one of the submirrors, or if a submirror consists of a single slice, you can keep the data on the slice.