Add a new server in admin area.

Then move all flv files to new server with ftp or sync.

Once this is done, update video table and change server_id of all local videos to the id of the server where the videos are uploaded.

Code:
update `video` set server_id=NEW_SERVR_ID where server_id=0
Take a backup of your database before doing this so if anything went wrong, can restore old db.