Hi Malte,
If we're talking ACS then the only thing you might be able to do (can't say for certain as I haven't done it myself) is to use a workflow to perform an update on the elements to associate them to a different campaign folder.
ACC allows a bit more flexibility with the folder structures but caution still needs to be taken in either instance since the elements could break if the folder paths and associated joins are not updated correctly.
Regards,
Craig