The web page implies you can try it immediately. Initially it wasn't available.

A few hours later it was in both the web UI and the mobile app - I got a popu[ telling me that GPT-4o was available. However nothing seems to be any different. I'm not given any option to use video as an input, the app can't seem to pick up any new info from my voice.

I'm left a bit confused as to what I can do that I couldn't do before. I certainly can't seem to recreate much of the stuff from the announcement demos.