So on OpenWRT for example, a WAN interface can be set get an IP address via DHCP, however, this will launch a separate independent process on Linux to act as the client. On the web interface, there is a button on each network interface which allows you to restart that individual interface (and in addition, it will be smart enough to kill the previous client process and launch a new one as well as re-initialize the firewall and other stuff). I could not find the equivalent command line option to this as a simple ifdown/sleep/ifup would not capture all of these sub-actions as per the extra configurations set on the interface. So I searched and read a helpful hint that can give a similar workaround using the network init startup script. You can modify some small part of the network interface configuration and then call network reload which will be smart enough to pick up the diff/changes only. This helps greatly in case you need to script or schedule a specific interface restart:

#!/bin/sh
r=$(/bin/date | /usr/bin/md5sum | /usr/bin/cut -c 1-8)
/sbin/uci set network.wan.hostname="owrt${r}"
/sbin/uci commit network
/etc/init.d/network reload