You can't change currency once the subsidiary is saved. In theory you can add another subsidiary with a different currency above your TOPCO, though this will require engagement with NetSuite and a special tool as well as some risks involved.
If you add another subsidiary above your TOPCO then you will have consolidated reporting in both the currency of your original TOPCO as well as the currency of the new TOPCO.
Alternatively, you can look into multi-book option and have reporting in another currency by utilizing the second accounting book.