Bitcoin could easily upgrade to 512 Bit security.
If someday we "upgrade" bitcoin i don't think it would be to a bigger size curve (which the bigger keys come from) but i suppose it would be a migration to another different asymmetric cryptography algorithm instead of elliptic curve and keep it small at possibly the same 256 bit key size.
Sure, I do hope so. I was only pointing out a very simple yet effective (not efficient, thou) defence against such machine is aldready avaliable: hence the threat from such scenario is not credible.