First of all, be better about keeping your private key secret, but second of all, NIP05 verification. If you use SN to verify your nostr, you can change the pubkey stacker news side and your friends will see that your old account no longer has that check mark.

Assuming you still have your privkey you can message everyone your new nostr pubkey

But also like, they're going to figure it out because you aren't acting like you anymore