To migrate content from one DSpace to another, you can export content from the Source DSpace and import it into the Destination DSpace.
Starting with DSpace 1.7, you can transfer any DSpace content (Communities, Collections or Items) from one DSpace to another by utilizing the AIP Backup and Restore tool. This tool allows you to export content into a series of Archival Information Packages (AIPs). These AIPs can be used to restore content (from a backup) or move/migrate content to another DSpace installation.
For more information see AIP Backup and Restore.
Where items are to be moved between DSpace instances (for example from a test DSpace into a production DSpace) the Item Exporter and Item Importer can be used.
First, you should export the DSpace Item(s) into the Simple Archive Format, as detailed at: Importing and Exporting Items via Simple Archive Format. Be sure to use the --migrate option, which removes fields that would be duplicated on import. Then import the resulting files into the other instance.
If you are using the XMLUI in both DSpace instances, you may also choose to enable the OAI-ORE Harvester. This OAI-ORE Harvester allows one DSpace installation to harvest Items (via OAI-ORE) from another DSpace Installation (or any other system supporting OAI-ORE). Items are harvested from a remote DSpace Collection into a local DSpace Collection. Harvesting can also be scheduled to run automatically (or by demand).
For more information see Harvesting Items from XMLUI via OAI-ORE or OAI-PMH