As you’ve noted, creatorId is an insert only field, which means it cannot be updated.
However, you can update the userId to any value you please. This will effectively make that user the entry’s owner, with full privileges.
A series of videos in our organisation were uploaded with an Admin_Group_ID as creator. The consequence is that every member of this admin group has these videos in their “my media” even when filtering by “media I own”.
The reason for my question is to remove unwanted access rather granting access, so that people can have a way to only see the media they’ve created.
It seems the only way is to delete these media and re-uploading with proper creator ids.
In that case, I believe a shorter path, if this approach is viable in your case and I am not sure that it is, would be to remove this group entirely and re-erect it with a slightly different name. Then, you could assign the correct userId per entry as I explained in my last response.
The reason I am not sure this is a viable for you is that I do not know whether you are using this group for other purposes. If you are, naturally you would not want to take this route.
Alternatively, you could try concating your account manager (I am making the assumption that you are a SaaS customer, as opposed to a Community Edition user) and inquire as to whether it would be possible for this change to be done by professional services. This is not normally done so I cannot commit to anything but asking can’t hurt. I should note that such a change request will incur additional charges.