Hi Vallis,
Thanks for reaching out! Unfortunately, there isn’t a straightforward way for automatically activating a specific ticket type once other ticket types have sold out. The best means for accomplishing this would likely be to publish your event with the correct stock amounts for your initial ticket types, then edit your event to add an additional ticket type once you’ve received email confirmation that all other tickets have sold out. Your e-commerce platform of choice should have an option for sending stock alerts via email to your administrator email address–if you’re not sure whether this feature is enabled for your site, I’d suggest reviewing the documentation for your preferred plugin.
I hope that helps to get you started here! Thanks again, and let us know if you should have any other questions! 🙂