Would the public key of Digital Signature be replaced?
extended and transformed comment into an answer

Digital signatures are not a complete system, they are one mechanism. They assume public keys are known by everyone.

In practice, this is often achieved by some sort of public key infrastructure. In software it's also possible the program has the verification key hard-coded.


  If there is a safe way to send a public key to receiver, why don't just send the message in the same way?


Well, that's pretty much what the chain of certificiates in a PKI do - but the sender might be a different person then.