If they did that, PN would be in breach of contract. They would have to keep to the terms of the old contract or product, or thats grounds for termination, for breach of contract.

Plusnet say bronze will (sometimes) go slower than gold in the traffic management queues, but they haven't said how much slower. In my opinion they could slow bronze on Extra Fibre as much as they want, legally.
Now, the Unlimited Fibre is a whole different ball game, because using the "unlimited" word carries certain obligations, namely: "Provider-imposed limitations that affect the speed or usage of the service are moderate only and are clearly explained in the marketing communication".
Of course, the word "moderate" is open to debate, but if Plusnet do "de-prioritise" bronze to a large extent, the ASA would be interested.
Oliver.