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This plugin adds a control to each forum discussion post. Only administrators see it and it is used to move a discussion to a different group. The display of the control can be turned off in the plugin's settings.
Two important points:
1. The admin can only move the discussion to a group the admin belongs to.
2. The people in the discussion may not belong to the new group and so cannot add to the discussion until they join. This works the same as if they left a group where they posted.
This has only been tested with Elgg 1.7
In there you can to find move post feature too.
Use it for moving ALL content (posts, files etc) from one group to another