If so, and if you are somewhat familiar with SQL and the DSpace database, I'd suggest you could make the changes directly via sql update in the resourcepolicy table in your database.
Disclaimer: Please take this approach with a grain of salt and test it on a single item first, preferably in your test environment / test database. I don't know if there is a solution via the user interface, so this is what I would do, but if someone knows a more simple and secure solution via the user interface, I would also prefer the latter!
update resourcepolicy
set rpname = 'administrator', rptype = 'TYPE_CUSTOM', epersongroup_id = 'uuid of your administrator group'
where resource_type_id = 2 and action_id = 0 and dspace_object in (comma separated list of your uuids)
this changes the existing access policies of your items to your administrator group only (you have to know the uuid of your administrator group, too).
If you are not yet able to identify your items exactly by uuid, it will be a little more difficult, as you will then have to work across several tables (item2bundle, bundle2bitstream) to find out which items do not have bistreams. I can't go into this in detail at this point.
Kind regards,
Matthias