in 1990 there would have been time to do this, in 2011 there isn't. part of this discussion is the compalints that the 'migration plan' was horrible, to which peopel are responding claiming that it's not horrible, it's the only possible way to do things. As a result people like me are pointing out things that people could do.
and the IP spec has been changed over time to include new features. there are reserved feature bits in the header that could be used to indicate the presense of these changes.
this is just plain going to be a horrible mess. there are few possible outcomes
1. we have a long period of using NAT and then move to IPv6
2. we have a long period of using NAT and then move to something else (probably at least as painful in the meantime as the move to IPv6)
3. extensive use of NAT becomes permanent
4. the internet as we know it collapses (extremely unlikely in my opinion)