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:

The Service Check Date is current

The device is eligible to install the intended TMOS version

Any module entitlement updates are received

Reactivation does not interrupt traffic and does not require a reboot, making it safe to perform before the maintenance window.

F5 best practices state:

Perform all non-impact tasks prior to the scheduled maintenance window

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

Since license reactivation is non-disruptive , it should be done before the upgrade window starts.