Unfortunately the PassKit Zapier integration does currently not support the "change tier" action, but no worries, you are still able to accomplish this with Zapier and the PassKit API.

You can change the tier by using the Zapier Webhooks app in combination with the change tier API method and the long lived auth token.

For more information and a detailed video on using Zapier webhooks with the PassKit API, please check out this article:
https://help.passkit.com/en/articles/4191331-synchronizing-digital-membership-cards-with-your-contact-records