Tested the DHCPv4 part with success.

With a correct setup (server routing, dhcrelay
with the 2 interfaces):

 - old ISC DHCP works but does not copy
  RAI relay port sub-option and sends replay to port 67
  (expected behavior, it works because the relay
   is backward compatible and listen on both the
   relay port and the legacy port)

 - new ISC DHCP works, copies the RAI relay port
  and uses the port it receives queries from.

 - new Kea works, copies the RAI relay port and
  uses the port it receives queries from
  (I didn't check old Kea but it should behave
   in a similar way than old ISC DHCP).

Tomorrow I'll try the DHCPv6 part.