I have (worked at a networking equipment company on IPv6 support, briefly). The few bits that are totally different (ARP-to-ND, for example) are all very small

I have (worked at a networking equipment company on IPv6 support, briefly). The few bits that are totally different (ARP-to-ND, for example) are all very small bits of code. The really hairy stuff was always the dual-stack - the assumption of a single IP instance running over any given layer-2 network is baked deeply into most systems, and changing that is a royal pain. That plus the renumbering of constants (ugh ICMPv6). The way I explained it to people was: IPv6 is exactly the same as IPv4, but with incompatible field sizes and constants, different (but usually MUCH simpler) address configuration, and no hop-by-hop fragmentation (oh frabjous day!).