Solaris

The best way of altering a host's name, IP address, domain, timezone,
etc., has always been /usr/sbin/sys-unconfig.

This command asks for a new system's configuration consists of
hostname, Network Information Service (NIS) domain name, timezone,
IP address, IP subnet mask, and root password. This command makes
sure there are no applications running that get the configuration
changed while they are running (by shutting down the OS), and makes
sure the configuration details are changed in all the right places.
Please read man page of sys-unconfig(1M) before proceeding.

Please also check that if there is a /etc/inet/ipnodes file exists and
if there is IPv4 ip address entry in that file for new hostname, then
that entry should be same as the one in /etc/hosts file.

Another method, which is not recommended is, changing hostname manually
in files:

/etc/nodename
/etc/inet/hosts
/etc/hostname.le0 or hostname.hme0 or an equivalent file
/etc/net/ticlts/hosts
/etc/net/ticots/hosts
/etc/net/ticotsord/hosts
/etc/inet/ipnodes (if file exists and if hostname entry exists)

After modifying the above files, reboot the system.

Above two methods can change hostname of system. But for other
operations depending on configuration, you may also have to change the
information in /etc/inet/ipnodes, DNS files, NIS server entries,
application files etc. having hostnames.