For an upgrade of a standalone BIG-IP, a maintenance window is available in which brief...

License reactivation updates the BIG-IP device’s license file to ensure:

TheService Check Dateis current

The device is eligible to install the intended TMOS version

Any module entitlement updates are received

Reactivationdoes not interrupt trafficand does not require a reboot, making it safe to performbeforethe maintenance window.

F5 best practices state:

Performall non-impact tasks priorto the scheduled maintenance window

Leave the window available for activities that require rebooting, such as the software installation itself

Since license reactivation isnon-disruptive, it should be donebeforethe upgrade window starts.