So what happens if you booked a higher class but it isn't available in the UC line? I know it's rather unlikely but I've heard some very chilling reports of people being charged for upgrades (not from the upgrade lot but from the gold desk) after they didn't get their reserved class (although I do believe there was a little more to the story).

I have an upcoming reservation for an F in BOS, booked long before UC was a thing. I've never used UC since I always had assigned vehicles (convertibles, mainly) but did strole through the Gold UC lot at LAX a couple of weeks ago and was somewhat underwhelmed. Naturally, I didn't expect any convertibles or V8s but it did look like it was 95% intermediates.

Iirc the upsell from an intermediate to a full size was pretty low, 20$ for the whole week or so, but uncompensated downgrades still aren't too high on my list.