I think what you suggested is the best thing to do.

Just make a new copystgpool, with a copy of your new prim.stgpool.
After that, you can delete the old copystgpool. Or if the databasespace is
the problem, and you can risk the very littlebit of risk: after migration
delete the old copypool and define the new one on the new library.

Or if you want to keep the copy on the dlt libr., copy both prim.stgpools to
the copystgpool. TSM don't make a new backup of your files when you migrate
the data.

A 3th thing you can do is a move data command. Move the data from 1 tape to
an other pool, but i'm not 100% sure that it could be done with
copystgpools. And it costs a lot of time and work to do all the commands.
(tape by tape).

Greetings,
   Maurice van 't Loo
   Compare Computers
   The Netherlands