If these repositories all exist in the same GitHub organization, then you should be prompted to upgrade when trying to add the project from the create projects page.
While some projects needed to stay open, our project need to be private for now. We have purchased and created a separate sonarcloud organisation, but pointing to the same github organisation with a view to reviewing only our private repos.
Hopefully we should be able to do this using github actions without losing any functionalities right if we decide not to add the github organization. Or what issues could we face?
This exactly what a single organization with both private and public projects will allow. Some projects can be visible to everybody, while some stay private. There should be no need for separate SonarCloud organizations, unless I’m missing something.