@markdudov
Release WAN : shuts down the DHCP client on WAN? and takes down the WAN interface.
Relinquish : also destroys the information about the current lease, so the next time WAN gets activated and DHCP client start, it will ask for a lease, and it will not ask for a preferred IP address, the one it had last time. The DCHP server can override the preferred request, and still assign another one.
When you "Relinquish", the next time the DCP client starts, it will as for any available IP without preferences.
Just my 'MHO' of course.