Hi Tom,

you leave us in darkness what your offsite pool exactly is.
If it is a backup storage pool of your "onsite" pool, 
then yes, deleting onsite files will delete offsite files as well.

Seems like what you want to accomplish is simply effectively turn
co-location on, for new and for existing data.

There are simple ways to achieve this, like following:
update the storage pool concerned to be co-located
be sure you have reasonably much free tape volumes in it
tape by tape, move data from existing volumes back either into same
storage pool,
 or in the originating primary disk storage pool.
 You will not loose a single file and you will end with collocated tapes.

regards

Juraj Salak