Is it possible to change a user's nickname using their ID? I tried for a long time to solve this problem, but I never succeeded.
Why don't you use Member#modifyNickname
?
Member member = event.getMember();
member.modifyNickname("Im a Maniac").queue();
Since you haven't provided code I can't tell if this snippet would work, it should if you refactor it as to your needs.