Changing version or reinstalling
The same operation for both. Your world, your plugins and your configuration stay where they are.
What happens
Reinstalling runs your template's setup again, against a version from the same list you saw when creating the server.
What gets replaced is the server software and the files the template itself owns. Your world, your plugins, your mods and every configuration file you edited are left alone.
That is why there is no separate button for changing version: going from 1.21.4 to 1.21.8 and repairing a broken server jar are the same operation on the node.
The server has to be off
Reinstalling only works on a stopped server. Replacing the server software under a running world is not a thing to do quietly.