Is it safe to upgrade a license on a running production cluster?

You can upgrade a license without taking down a production cluster.

If you are using a CSN, you can simply go to the Licensing page (Content Storage/ Licensing) and upload your new license.

If you aren't using a CSN, then it depends on where your license file resides. If the license is on your USB key you simply remove your key, change the file (use the same filename) and reinsert the key. If using a different PXE server, simple replace the key where it resides on the server (using the same filename).

Your nodes should notice the new license within about 15 minutes.