If you want some files in a different tape storage pool, you will need a new
management class for them.
If you are backing up direct to tape, just point the new management class to
the new tape storage pool.  Use INCLUDE statements in dsm.opt to bind just
the files you want to the new management class.

If you are backing up to disk first, you will need to create a new disk pool
for these files as well as a separate tape pool.  (You can't have one
primary disk pool that migrates to two different tape pools.) Make the new
management class point to the new disk pool and have the disk pool point to
the new tape pool.

When you backup files to the new management class, the NEW backup versions
will go to the new pool(s).

Nothing happens to older/existing backup versions of those files.  They stay
on whatever media they were on originally.    Nothing triggers them to move
or relocate.  They aren't hurt, you can still restore from them if you need
to.  The best thing to do is just leave them where they sit, and let them
expire when they normally would.