no, but i heard nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s say that something might be in the works for this

No

NO - u can use multiple npubs - also many handles

every npub has its own key for signing

only issue will be people who know u by old npub may get confused to know real one

You would just create a new one.