Although Service Portal was designed to address many of the use cases and pain points of CMS, they are entirely different products.
Nothing - all your CMS projects will be completely unaffected by Service Portal. Service Portal is not an "upgrade" of CMS, it is an entirely different product.
First, the bad news: there is no direct migration path from CMS to Service Portal.
Now, the good news: Service Portal comes with widgets for many components such as Catalog, Knowledge, and Incidents. These components should work as you expect out of the box. Additionally, the Service Portal developer framework makes component development much easier than it was in CMS, which means that you will be able to build any functionality you had in CMS portals into Service Portal with relative ease.
The difficulty of a migration from CMS to Service Portal will depend mostly on the customizations made to the components in CMS.