That's called 6to4 and already works nicely
It's for people who only have an IPv6 connection to connect to IPv4 hosts. AFAIK it uses some sort of NAT mechanism for this. It cannot be done directly as the legacy host would only get the truncated address and therefore couldn't reply.
Your suggestion would essentially be the same as IPv6, but with much shorter addresses. You'd still have all disadvantages of the switch, but without any of the advantages.