I'm a ChatGPT Plus Subscriber and I just refreshed the page and it offered me the new model. I'm guessing they're rolling it out gradually but hopefull it won't take too long.
Edit: It's also now available to me in the Android App
You can use GPT-4o now but the interactive voice mode of using it (as demoed today) releases in a few weeks.
i can try it now, but now the voice features i dont think