Can I upgrade from POWER Starter to Pro theme?

Not directly, the code base for POWER Starter is not the same as the one for POWER Pro

It is not possible to use the templates and modules from one theme with the other.

If you have purchased and built your site with POWER Starter and are considering moving to POWER Pro, this would essentially require a website migration.

There are many similar features and capabilities which would allow you to style the two themes to have the same appearance, allowing you to transition the website internally without an impact on your site visitors.

There are differences in the header and footer that would prevent you from using all of the Pro features until you've fully migrated to keep a consistent user experience.