Change TZ globally from script?

We’re setting up for the extended daylight savings time season next year, which means changing our timezone information.

Changing the settings in sysinit is straightforward.

What I’d like to know is if there is a way to the change timezone variable TZ on a running PC globally, say from a shell script. So far I can only make the change to the active session: when I log off the update goes away. I’d like to avoid having to reboot if at all possible.