We went through a similar migration at my last company.  There is no way to
accomplish this that I know of.  You need to just start backing up to the
3494
and wait for the old tapes from the 3575 to expire out of inventory.  You
also
need to make sure that you still have the 3575 available to you until all
the
tapes files on thos tapes have expired.  This all entails setting up new
storage
groups also.  Sorry for the bad news.

Richard