I’d unplug the old hub and stop the sky (opening battery door) before installing the new one with the Tempest as the app sees all hubs and by accident you could link the Tempest to the old hub. By unplugging the old hub this can’t happen. Once done setting up the new hub and Tempest, you can then reboot the sky and reset it by a long push on the led (wait it starts to blink green again) and add it to the new hub (it should see it).
regarding sky calibration, yes it will be lost as it will be seen as a ‘new’ device. Support can migrate the calibration but not sure it will be quickly since they are still struggling to catch up.
In worst case it will take a few weeks to come back to it’s calibration (except wind that will be ok after a few minutes and wind from all directions)