Accidentally I clicked on “I know All”, now all my unknown Chinese characters are not blue any more. I tried to “Change Status -1” but nevertheless they are shown as “known”.
(even when I edited, removed and copied again). How can I “move them to unknown”?
You can’t reverse this action. However, if you highlight any words you don’t know, you will be able to then hover on the highlighted word and create your LingQ. This will remove that word from your Known Words list.