[hot_gril]

It would still be a much smaller shift if they focused on only expanding the address, and I guess removing packet fragmenting while they're already changing the fields. Why not do it right, cause at least it gets done that way.

There have been other huge migrations pulled off in networking. HTTP->HTTPS is the first that comes to mind. Extra layer of security, no other changes. Browsers slowly made users more and more wary of unsecured sites, and it became easier for site admins to obtain SSL certs. Once plain HTTP was finally made uncommon, versions of SSL/TLS still got upgraded slowly. They also avoided making it too flexible and turning into a fragmented mess like email or XMPP, i.e. browsers strongly avoid self-signed certs and started banning old versions.