> Torrents are immutable in principle
In practice, that's mostly how they're being used.
But the protocol does support mutation. The BEP describing the behavior even has archive.org as an example...
> The intention is to allow publishers to serve content that might change over time in a more decentralized fashion. Consumers interested in the publisher's content only need to know their public key + optional salt. For instance, entities like Archive.org could publish their database dumps, and benefit from not having to maintain a central HTTP feed server to notify consumers about updates.
http://www.bittorrent.org/beps/bep_0046.html