Having the threshold at 0 during client backups is not a good idea (as
you've learned).  You want to perform the migration during the day, while
there are no client backups running.  The worst of this, is setting the
threshold backup up while the client backups are waiting for tapes won't
change anything.  You're stuck until a) the backups complete, or b) you
kill them all with cancel session.

Kelly Lipp
Storage Solutions Specialists, Inc.
lipp AT storsol DOT com
www.storsol.com
(719) 531-5926