I don't think it changed? I think it was just a flashback of a memory which kyon does not know of ?
It looks like in that split second he was touched the mark changed and he was controlled to attack Lucaon. He had that blank look on his eyes but snapped out of it when he saw Lucaon got hurt.
_(:з」∠)_Maybe Locan is a royal blood,too.And he put the mark when he put his hand on kyon's neck.Then kyon is so afraid because he think zero will hurt him.That's why Kyon fought back with the knife,but didn't expect he is Lucaon,isn't it?
(*/ω＼*)Sorry my poor English…
Idk if this is true but I saw someone on ig posted that the mark on kyon's neck changed since ch. 1. Has anyone notice? Is this true? Can anyone tell me on which specific chapter it changed?
(NO SPOILERS)