Power Apps portals and Dynamics 365 portals are now referred to as Power Pages. There's no migration or upgrade process. You can use the new Power Pages design studio to edit sites created in Power Apps. Existing tools such as the Portals Management app and Power Platform CLI work with Power Pages.
Microsoft Power Pages is a secure, enterprise-grade, low-code software as a service (SaaS) platform for creating, hosting, and administering modern external-facing business websites. Whether you're a low-code maker or a professional developer, Power Pages empowers you to rapidly design, configure, and publish websites that work across web browsers and devices.
Power Pages provides rich, customizable templates, a fluid visual experience through the design studio, and an integrated learning hub to quickly build sites for your unique business needs.
Power Pages is the newest member of the Microsoft Power Platform family. With Power Pages, you can build sites by using the same shared business data stored in Microsoft Dataverse that you use for building apps, workflows, intelligent virtual agents, reports, and analytics with other Microsoft Power Platform components in your organization.