They could have made the change backwards compatible. Basically by doing something along the lines of designating a part of IPv4 space for use with the new protocol, and then stuffing the extra address into the optional header field. Then if you're running an older device that doesn't understand the new header it will just forward it along until it gets to a device that can.
Which puts you in the same problem: to any device (i.e. one with a hard-wired ASIC - the largest and fastest hardware generally) the new address space does not exist.
The internet goes to some lengths to avoid routing loops as well, so once a packet is passed one router we're not going to be able to easily spin it around and send it back to get to the right place. The net effect is going to be to randomly DDoS a bunch of devices which happen to be behind non IPv4.5 hardware, and so receive 50% of the internets traffic which doesn't get routed properly by an upstream router.
