Hah! I learned this the hard way a few weeks ago... I updated my Wi-Fi password. Every-god-damn-"smart"-piece-of-crap in my house went offline (as expected).

Then I learned there was no way to give the new Wi-Fi password to any of these devices without manually re-pairing them in HomeKit and reconfiguring all scenes and everything else...

Like, that seems like basic functionality Apple should have included in the HomeKit design.

In the end, I gave up and just put the Wi-Fi password back.