Upgrading existing ArcGIS Open Data sites does not transfer the items and datasets automatically. The number of groups are shown on the Site Manager page but zero items and datasets display if there are no changes made to the upgraded site.
Option 1: Go to 'Site Editor' and click the Save option. Go back to the Site Manager page, and the items and datasets are displayed.Option 2: Make changes in the URL of the upgraded site, and the Save button is enabled to save the site. Go back to the Site Manager page, and the items and datasets are displayed.