If you're on a minimum term contract for app access at the moment, you'd need to see this out before cancelling as per the terms and conditions.
If it's a rolling monthly contract, you'll need to cancel so it stops at the end of the billing cycle.
The BT TV pack would involve a new minimum contract period but you will be able to access BT Sport via the app and STB.
Unless it's changed, the basic subscription is SD only so you'll need to add HD to the monthly subscription and if you're line speed is fast enough you'll also get access to BT Sport Ultimate and Ultimate HDR.
Oh and you also need to have your broadband from BT 👍