in short if the hash function is to be replaced it must be replaced with something different. for example one of those sponge based constructions instead of Merkle–Damgård, like Keccak (standardized as SHA-3).
And this I guess is why when we're seeing proposals, they're for different/new hash functions? Although until I saw this I didn't know about this SHA-3 family of standards. I don't trawl all the proposals but that's what I recall (or don't recall) but I always thought that when, if at all, Bitcoin decides for a major change, it will only be to hash funtion, not hash algorithm.
Are you aware of anything suggesting otherwise?
yeah, SHA-3 family have been around for a while now (2015ish). there are even a lot of different altcoins that use these functions. some examples:
Blake2 (BlakeBitcoin),
Grøstl (DigiByte, Verge),
Skein,
JH (SecureCoin), and of course
Keccak (Ethereum)
i don't think anybody is even thinking about replacing SHA256 at the moment. there is just no reason for it. this hash function is still strong against all the attacks that matter.