I’m pretty sure the cryptographic parameters to generate that public key are included in the private key file. So while you can generate the other file from that file, it’s not only the private part in it and you can’t really change the characters in the private key file. Also not an expert here. I’m fairly certain that it can’t happen the other way round, or you could impersonate someone and do all kinds of MITM attacks… In this case I’ve just tried changing characters and openssh-keygen complains.