Rename VLANs

Yes. This would be traffic impacting because you would have to reload the configuration files in order to load the changes.

During the configuration reload all traffic would be disrupted.

If you are going to do this I would recommend doing it during a Maintenance Window (or on an HA Pair, Sync the Pair, perform the changes on the Standby device, save the changes, then perform a "b verify load" / "tmsh sys config load verify" (if there are no reported errors then you can load the config), initiate a failover, verify that everything is running properly...then rinse and repeat.

Hope this helps.