What was Félix’s promise exactly? Was it a self promise or did he promise Adrien?
Félix’s first promise to himself he would protect Paris and Ladybug, for the rest of his days, until Hawkmoth was defeated.
The second thing he promised himself was that he would help heal Adrien’s friends hearts from the tragic event that happened to Adrien, because he knows that Adrien wouldn’t want his friends to be sad because of him.
His third promise was to not fall in love for anybody. Which he was doing a very terrible job at keep that one, since he fell for Marinette before the tragedy. (That would be explained in a different dialogue) He knows if he gets attached, it would be really hard not to spill the dark truth about himself, since he would want to be fateful to his lover. But he is unsure if she would ever love him back anyway, since she likes Adrien and even if she did move on from him, his dark secret would lead to more heartbreak for her. But love can make you do crazy things, so he will be back and forth with this promise specifically.
His last promise was when Hawkmoth and Mayura are defeated, he would announce to Paris his sins and how he her really got his Miraculous. (Since no one knows Félix stole the Miraculous, except Master Fu) He knows what will happen after he tells the truth and hopes Paris would forgive him one day. It would be the day he no longer has to be Chat Noir.
And that’s how he purposely transferred schools and became the new Chat Noir as a result. Hope this helps. -KT