IOS – what happens if I update unlocked iphone software

So in the end I had to do the SIM card dance.

In fact, it seems what the carrier does is mark your phone as allowed for unlock.

Then, the first time you put a SIM card from another carrier, you must connect to iTunes, which will then perform a series of annoyances and alerts and whatnot, and only then your device will be truly unlocked. You need an internet connection for this process.

I know this is a terrible answer to my own question.

It won't affect the unlock. A legitimate (ie sanctioned by the carrier and not hardware/software) unlock happens at the carrier level and jailbreaking doesn't affect that.