There is no online method I know of unless you are moving an entire volume (which will migratethe qtrees) with Data Motion for Volumes (ONTAP 8.0.1 with LUNs only, not NAS) in the same controller, or Data Motion for vFilers (ONTAP 7.3.3+ , not 8.x yet) which moves all volumes in the vFiler. To move an individual qtree, Qtree Snapmirror (QSM) with a planned cutover (short amount of time to cutover) is what is most often used.