Changing the hostname at a Windows Server will require a reboot. However, changing a hostname on a Unix/Linux server do not require a reboot! But, it is not so straightforward to change the hostname of a server online. This is because you will need to change a couple of things, like the entry in ODM, nodename and current hostname.
oldhostname:/:>chdev -l inet0 -a hostname=newhostname
oldhostname:/:>uname -S newhostname => Change the network name of this machine.
oldhostname:/:>hostname newhostname => Sets the name of the current host system.