It might also be necessary/good idea to delete any lease file(s) related to the interfaces which is not specified as dhcp-assigned in the interface config file.
It seems, even though you kill the dhclient process, it might come back if you don't delete the lease file. In my view this issues is very common and there you could argue it should be escalated to a bug and not a feature request. I will claim that the majority of all new installations get an dhcp assigned IP address and the majority of all server-setups will be changed to static....and then we have the problem, that's a lot of pain out there! Perhaps most just restart but you should be able to just do a "ifdown --all; ifup --all" and then everything is good, but that's not the case. (well not quite: when I change a new servers to static and add a bridge interface, "ifdown --all; ifup --all" seems not to enable the bridge interface. I have to do "systemctl restart networking" too, but fixing this dhclient issue would be a good start)
