We've transitioned a site from Omeka.net to a hosted Omeka.org site, and in the transition Items were no longer categorized into Collections and Exhibits.
Is there a bulk way to re-categorize all of our items? Or do we need to go into each Item and re-cateorgize them into Collections and Exhibits?
For the Collections, on the admin item browse page you can do bulk actions to put them into Collections. Just check the boxes and click the Edit button. If you have a lot, you might want to increase the number of items displayed under Appearance -> Settings
Exhibits are another story. It's likely that you'll need to rebuild the Exhibits in your new installation from scratch.
Out of curiosity, in your new omeka.org site, what version are you using? It'll help us to know more about what version of Omeka people use when they move from Omeka.net to an omeka.org installation. And, if it is Omeka 2.x, whether you are moving to the newest Exhibit Builder 3.0