Well, "re-initializing" an existing Gerrit installation is no big deal. You usually just confirm the existing settings, which takes a minute or two.
In batch-mode (with the
--install-plugin parameter), even faster. Of course you would need to stop the server before doing that.
If you don't want to touch your production instance, you can always just do another "install" in another folder (or even another pc), install the plugin there and copy it over to your production instance.
Just make sure to use the same version (gerrit-3.6.1.war).
Gerrit, by default, checks the plugins folder every minute, so it should be recognized automatically after a short time.
Hope this helps,
Dennis