Move all models you want to export into one group, then export that group into an XML file, and import that one into the new database. Alternatively, export each group.
Use Merge by drag'n'drop all models to the new database. Drop them on the database icon NOT on a group, because ARIS also transfers the relevant group structure from the source database. And you will be prompted for some merge options. Open the Help in that dialog to learn about their meaning/consequence.
