RESOLUTION
Plesk 8 migrates only data from horde.turba_objects table, but does not migrate horde.horde_prefs and other tables which contain personal address book and other settings like filters, etc.
The data is not migrated because Horde does not provide any tools for import/export these settings and the Horde database format much differs from version to version. The ways to import and export these preferences are going to be closely investigated and implemented in further Plesk versions.
This time, if you need, you can try to dump and migrate Horde database manually. If Horde versions on the source and destination servers differ, the database should be upgraded after restoration on the destination server, please follow Horde documentation for details at http://www.horde.org/horde/docs/?f=UPGRADING.html
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