If you really wanted you to could keep you old addressing scheme in IPv6 (arbitrary IPv4 addresses can be embedded into IPv6), you can disregard all best IPv6 p

If you really wanted you to could keep you old addressing scheme in IPv6 (arbitrary IPv4 addresses can be embedded into IPv6), you can disregard all best IPv6 practices about subnetting and do everything like you did for IPv4, DHCP and all, heck even NAT. The problem is that as soon as you're turning it on you need to maintain two sets of routing and firewall configs, even if they were identical. Also you need proper support from all those lazy vendors (both hardware and software) that did the absolutely minimal amount of work to advertise their products as IPv6 ready when it fact the support ranges from subpar to practical unusable. As soon as you make a one bit incompatible change to the protocol routers aren't able to communicate anymore: it's the same situation again. It doesn't matter how similar the two protocols are, they're incompatible.