the only reason for even considering a replacement for the hash algorithm (SHA-256) is if there were any security risk in using it. for example in the future maybe finding a collision for SHA-256 became as easy as finding a SHA-1 collision today. in which case the replacement should be to a different algorithm not the exact thing (ie. SHA-512).

lets not forget that if we change SHA-256 to SHA-512 we also have to probably change the elliptic curve to a 512 (or 521) bit curve. and that would hugely increase transaction sizes as both hashes and signatures would take twice the previous space.