This is outdated information. These issues have been solved by various BitTorrent Enhancement Proposals. You do create a new torrent, but you distribute it in a way that to a swarm member is functionally equivalent to updating an old torrent. Check out BEP-0039 and BEP-0046 which respectively cover the HTTP and DHT mechanisms for updating torrents:
If that updated torrent is a BEP-0052 (v2) torrent it will hash per-file, and so the updated v2 torrent will have identical hashes for files which aren't changed: https://www.bittorrent.org/beps/bep_0052.html
This combines with BEP-0038 so the updated torrent can refer to the infohash of the older torrents with which it shares files, so if you already have an old one you only have to download files that have changed: https://www.bittorrent.org/beps/bep_0038.html
https://www.bittorrent.org/beps/bep_0039.html
https://www.bittorrent.org/beps/bep_0046.html
If that updated torrent is a BEP-0052 (v2) torrent it will hash per-file, and so the updated v2 torrent will have identical hashes for files which aren't changed: https://www.bittorrent.org/beps/bep_0052.html
This combines with BEP-0038 so the updated torrent can refer to the infohash of the older torrents with which it shares files, so if you already have an old one you only have to download files that have changed: https://www.bittorrent.org/beps/bep_0038.html