It is generally advisable to only modify licenses during a maintenance window.
The only time where it is known to potentially cause an outage is if the licensed core count changes as a result of applying the new licenses.
This happens in a few specific circumstances:
- Moving from 15-day PnP to 30+ day eval licenses
- Moving from eval license (either 15-day PnP or 30+ day eval) to permanent license
- Changing Open Server licenses (e.g. moving from 2-core to 4-core or similar)
In the above scenarios, a reboot is required after applying the new license.