I'm aware that it's advisable to rotate cryptographic keys periodically. HKDF accepts high-entropy input (IKM - Input Key Material), and expands it to an arbitrary length. It also accepts a salt, the RFC states that "the use of salt adds significantly to the strength of HKDF". Naturally, changing the salt results in distinct Output Key Material.
What I would like to clarify, is whether updating the salt (in the context of HKDF) constitutes 'key rotation', or whether the IKM itself needs to be changed periodically. I suspect the latter, but I'd like to hear what other people have to say. I assume that this also applies to other KDFs like Scrypt.