IPv8 does not require dual-stack operation. There is no flag day. 8to4 tunnelling enables IPv8 islands separated by IPv4- only transit networks to communicate immediately.
How is this different from IPv6? We've had 6to4 for ages, the problem is the other direction: how does a IPv4 host initiate a connection to a IPv8 host?
Existing IPv4 applications use the standard BSD socket API with AF_INET and sockaddr_in. The IPv8 compatibility layer intercepts socket calls transparently -- the application has zero IPv8 awareness.
Except many IPv4 applications use the addresses of the source or that they bind to in some form. If it's secretly an IPv8 behind their back that'll break.