Piotr,
According to our checks engine succeed connecting VDSM through ipv6 once the work you've reminded has been done. So it looks like jsonrpc is agnostic to IP layer it uses underneath.
Could you please elaborate on why do you think that we break backward compatibility.
If you compare the wiki to vdsmapi-schema.json file, looks like the last isn't updated for some time and needs to be updated according to the actual code.
We add dhcpv6 in order to allow user to configure v6 stack independently from v4 one. That could be useful especially in a transition period, when a user have fully configured ipv4 and would like to try using ipv6 and he hasn't configured his DHCP server for v6 yet.